BUG: Don't write to the fs on module import.

[tswast]

Avoid writing to the filesystem when the module imports.
This prevents errors when the home directory is
unwriteable.

Closes #10.

CC @PeterJCLaw

[tswast]

Tests pass locally.

$ pytest tests
======================= test session starts ========================
platform darwin -- Python 3.6.4, pytest-3.7.1, py-1.5.4, pluggy-0.7.1
rootdir: /Users/swast/src/pydata-google-auth, inifile:
plugins: pyfakefs-3.5.6
collected 9 items

tests/system/test_auth.py .....                              [ 55%]
tests/unit/test_auth.py ..                                   [ 77%]
tests/unit/test_cache.py ..                                  [100%]

==================== 9 passed in 12.42 seconds =====================