API: print/show_backends

like `print_versions`: https://github.com/ContinuumIO/blaze/blob/master/blaze/__init__.py#L71

new function to print the backends (and evenutally their capability). personally I prefer `show`, but looks like `print` is in:

e.g.

```
b.print_backends()
```

```
Blaze Backends
----------------------
pandas [ 0.14.1, compute ]
mongo [ not-installed ]
sql [ 0.8.3 (sqlalchemy), compute, writable, readable ]
```

Start by reading blaze/__init__.py around the print_versions function referenced in the issue. Trace how Blaze backends are discovered and what version or capability information is available. Done means adding a print_backends API with output covering installed and unavailable backends and their relevant capabilities, as illustrated.

Written by the indexing model from the issue text.