Really make 'gflags' optional.

If it can't be imported, then we don't need to fix it up inside
'CredentialsFromFile'.
Use 'hasattr()' rather than 3-arg 'getattr()'.

Incorporates feedback from @craigcitro. See:
https://github.com/craigcitro/apitools/pull/11/files#r20564006.
Merge pull request #11 from tseaver/scrub-non_optional-gflags

Really make 'gflags' optional.