ipfs get cross platform paths

`ipfs get` does not handle paths with platform specific separators in them gracefully.

Consider the structure

```
test (QmSqqKkPViWCn5c6LShAqved78NtYr5okY8gN2frkNzJ9E)
|-- Directory with slash\ in it
| |-- Filename with slash\ in it
| `-- Regular filename
`-- Directory without slash in it
|-- Filename with slash\ in it
`-- Regular filename
```

`get`ing this on a system that uses `\` as a path separator results in an error. Here's an example on Windows with `\`.
[Note: github doesn't like this image for some reason, hosting via ipfs]

@whyrusleeping mentioned this in PR https://github.com/ipfs/go-ipfs/pull/1933#issuecomment-158579349 for `add` but it shouldn't be possible to add an invalid path on your platform so it was a non issue there, that's not the case for `get` though.

To handle this there would need to be some table of restricted symbols for various file systems/schemes/whatever, said symbols would need to be replaced with some other valid symbol. Maybe try to use the given path/name and fallback to filtering on errors like these? This would tamper with the validity of the objects you're getting, they wouldn't `add` up to the same hash but since the hash already exists and you already have it, pinning it directly should make this a non-issue.
Outside of Windows restricting `\` I'm not sure what else if anything is effected.

A way to workaround this is to circumvent `get` by using the gateway and external tools that do this conversion, for example using wget
`wget -r http://127.0.0.1:8080/ipfs/QmSqqKkPViWCn5c6LShAqved78NtYr5okY8gN2frkNzJ9E`
This will percent encode the offending slash.

Paging: @rht