ENH: pd.read_feather/to_feather
conda packages are now available [here](https://github.com/wesm/feather/issues/134)

so this should be straightforward. a couple of things to note.
- should support the `LocalPath/PathLib` objects (easy as we do this in other `read_*`)
- non-supported on windows ATM. need to skip for testing. `feather-format` will just raise an `ImportError` for now.
- catch and re-raise for non-supported things, mainly object dtypes that are not all strings. Alternatively / maybe better to introspect these columns (via `lib.infer_dtype`) to prevent in the first place
- test unicode support / encoding (not sure supported as of yet)
- biggest issue is how / if to support `Series` and a `DataFrame` with a non-default index. As we need a facility to write/read meta-data within feather [see here](https://github.com/wesm/feather/issues/74). Both these can simply `.reset_index()`, but the problem is they can be stored but we don't know on read-back what to `.set_index()`. Can just raise for now.
