Based on code from eventlet, but heavily refactored and modified:

- Split host parsing into its own class, re-used from the test case
- Hosts handle PTR lookups for ipv4
- Some bug fixes around what should be strings where.
- Allow strings for the rdtype paramater
- Watch last modified time instead of reloading the file at fixed
  intervals.

More of our DNS tests pass now (notable TestEtcHosts is now enabled
for this resolver), and we tend to generate more conformant results
than the ares resolver (by observation, not tested).