Change Makefile 'make test' to only use basic tests.

 * This should reduce the load on TravisCI workers.
Update test_gnupg.py logger settings.

 * The logfile is now appended to, rather than rewritten, so that we can
   still see past test results.
Update test_gnupy.py to use the same standard key generation params.

 * Test method now matches the batch key generation parameters in gnupg.py.
 * Fix trailing whitespace in test_gnupg.py.
Shorten assertion string in gnupg.GPG.__init__().
Change _is_file to return booleans and never raise anything.
Update docstrings in several functions.

 * Including updates to:
   _underscore()
   _hyphenate()
   _is_allowed()
Update list of allowed gpg options.
Rewrite function _sanitise().
Rewrite checks in gnupg.GPG.__init__().
Rewrite method gnupg.GPG.make_args() to use lists for _sanitise().
Cleanup the docstring and code in gnupg.GPG.list_keys().
Update test_gnupg.py unittests to reflect changes in the code.