Use gevent to parallelize storage operations in get_tags

This is a band-aid for #614

For context, hosted from a GCE instance with GCS being the backing
store, With this patch, we are able to 'docker pull' 859 tags without a
timeout.  Prior to this patch, we were *unable* to pull anything from a
repository with 200 tags.  This is the extent of my investigation thus
far.  Unfortunately, this change is probably just a constant factor
reduction in run-time as total network round-trips still grows linearly
with tag count.
fixed link to advanced docs in faq
Merge pull request #671 from bobrik/patch-1

Fixed link to advanced docs in faq
Merge pull request #670 from chuegle/master

Fixes issue with s3/boto/gevent hanging.
Use the fully qualified class name of the boto calling format.
Merge pull request #676 from justinjohnsonio/justinjohnsonio-calling-format-classname

Use the fully qualified class name of the boto calling format.
Add DRIVERS.md for implemented docker-registry drivers
Merge pull request #677 from tobegit3hub/master

Add DRIVERS.md for implemented docker-registry drivers
Merge pull request #678 from chuegle/master

Changes to enable eu-central-1.
Add docker-registry-driver-elliptics in DRIVERS.md
Merge pull request #679 from tobegit3hub/master

Add docker-registry-driver-elliptics in DRIVERS.md
Updated changelog and version bump
Add table of contents to README.md

To help quickly find information in the README
Fix quicklinks
Typo
Merge pull request #644 from bshi/gevent-get-tags

Use gevent to parallelize storage operations in get_tags
Merge pull request #684 from rtrauntvein/patch-1

Add table of contents to README.md