Filter disk, io & network metrics by device

This pull request allows us to configure a `device_blacklist_re` in datadog.conf. This will be checked against all disk & io devices so we can easily filter out fake devices.

This pull request contains a few things:

- device filtering on disk metrics
- device filtering on io metrics
- device filtering on network metrics
- run unit disk metric parsing tests on all platforms (since the parsing code is not os specific)
- removes a bunch of base exceptions
- random little cleanups
- added a platform library for cleaner platform detection

@remh @elijahandrews @alq666 can you review?