[LIBCLOUD-613] Allow user to pass "gp2" for "ex_volume_type" argument to the create_volume
method in the EC2 driver.

Reported by Xavier Barbosa.
Fix some pylint violations.
Add pylint config and pylint tox target.
Customize HTTP vendor prefix for Google Storage

Google storage uses/expects custom headers like meta data
and storage class to be prefixed with "x-goog" instead of
"x-amz". This enables use of object tagging in the Google
Storage provider.

Closes #356

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Fix link.
Add a note about retrieving object meta data in the Google Storage driver.
[Libcloud-589] Add a new compute driver for ProfitBricks provider.

Closes #352
Re-generate supported providers and methods table.
Merge remote-tracking branch 'upstream/trunk' into LIBCLOUD-611-support_create_image
[LIBCLOUD-661] Added ex_create_image for the GCE compute provider - Fixes to docstring after pull request review