menpo.io add pickle support, move to pathlib

Four things in this PR:

1. Added `menpo.io.import_pickle{s}`, `menpo.io.export_pickle`
2. Moved `menpo.io.base` to use `pathlib.Path` objects
3. Flipped exporters so signature is `(obj, fp)` instead of `(fp, obj)`
4. Removed three unneeded functions from `menpo.io.base` (they are only needed in `menpo3d`)

I've updated tests but not yet added new ones for pickle. Note that menpo3d will need a PR to be brought inline in terms of exporting style and using `Path` objects.