gen: support for channels

Channel support is probably a ways off, but I guess it's good to make an explicit issue of it ;)

Building the package

```
package channel

func Channel() chan int {
return make(chan int)
}
```

fails when built with `go build channel.go && gopy gen`. The following message is generated:

```
panic: unhandled obj [*types.Var]
type [&types.Chan{dir:0, elem:(*types.Basic)(0x53fac0)}]

goroutine 1 [running]:
github.com/go-python/gopy/bind.(*symtab).addType(0x8205d33c0, 0x8820778b78, 0x820623db0, 0x8820778b18, 0x8205d2fc0)
/Users/phil/code/go/src/github.com/go-python/gopy/bind/symtab.go:447 +0x522
github.com/go-python/gopy/bind.(*symtab).processTuple(0x8205d33c0, 0x8205d2fe0)
/Users/phil/code/go/src/github.com/go-python/gopy/bind/symtab.go:349 +0x149
github.com/go-python/gopy/bind.(*symtab).addSymbol(0x8205d33c0, 0x8820778408, 0x820623d10)
/Users/phil/code/go/src/github.com/go-python/gopy/bind/symtab.go:330 +0xd0a
github.com/go-python/gopy/bind.(*Package).process(0x82064af00, 0x0, 0x0)
/Users/phil/code/go/src/github.com/go-python/gopy/bind/package.go:179 +0x26dd
github.com/go-python/gopy/bind.NewPackage(0x820623c70, 0x820663880, 0x16, 0x0, 0x0)
/Users/phil/code/go/src/github.com/go-python/gopy/bind/package.go:44 +0x390
main.newPackageFrom(0x8205ea0a0, 0x1, 0x1, 0x82064adc0, 0x820664000, 0x8205ea0a0, 0x0, 0x0)
/Users/phil/code/go/src/github.com/go-python/gopy/gen.go:209 +0x3bf
main.newPackage(0x7fff5fbffa41, 0x1, 0x8205d0104, 0x0, 0x0)
/Users/phil/code/go/src/github.com/go-python/gopy/gen.go:179 +0x638
main.gopyRunCmdGen(0x820634a20, 0x8205cc260, 0x1, 0x1, 0x0, 0x0)
/Users/phil/code/go/src/github.com/go-python/gopy/cmd_gen.go:75 +0x588
github.com/gonuts/commander.(*Command).Dispatch(0x820634a20, 0x8205cc260, 0x1, 0x1, 0x0, 0x0)
/Users/phil/code/go/src/github.com/gonuts/commander/commands.go:221 +0x7e2
github.com/gonuts/commander.(*Command).Dispatch(0x820634c60, 0x8205cc250, 0x2, 0x2, 0x0, 0x0)
/Users/phil/code/go/src/github.com/gonuts/commander/commands.go:187 +0x1f9
main.main()
/Users/phil/code/go/src/github.com/go-python/gopy/main.go:42 +0x1fe
```

One potential way to handle this would be to equate channels and [multiprocessing.Connection](https://docs.python.org/2/library/multiprocessing.html#multiprocessing.Connection) objects. When a channel is returned by a Go function, gopy calls [multiprocessing.Pipe](https://docs.python.org/2/library/multiprocessing.html#multiprocessing.Pipe), and passes one `Connection` to a new listening thread and returns the other to the calling process. When the listening thread receives a value through its `Connection`, it calls a Go function that passes the value to the equivalent underlying channel.

The same strategy could be used in the other direction for passing a channel/`Connection` to a Go function from Python.

It might also make sense to create some class `class ChanConnection(multiprocessing.Connection)` which stores an explicit reference to the underlying channel, so that in the case where a Python routine passes a channel created in Go back to another Go routine, values passed through it don't need to pass back and forth through Python.