Add ListImages() support and acceptance test.

This is needed to support a (currently broken) acceptance test for
creating a cloud server.  Without the ability to auto-detect an image, a
human operator is required to specify an image manually, which defeats
the value of an automated test.
Merge pull request #39 from rackspace/list-images-4

Add ListImages() support and acceptance test.
Merge branch 'master' into create-server

Conflicts:
	interfaces.go
Add ListFlavors() and acceptance test.

This is required to support server creation acceptance testing.  Without
the ability to detect flavors automatically, it isn't possible to
automate acceptance testing, as then a human operator would be required
to manually intervene while testing.
Merge pull request #40 from rackspace/list-flavors

Add ListFlavors() and acceptance test.
Merge branch 'master' into create-server
Finish acceptance test for CreateServer() function

Two problems remain -- server status is coming up blank, and
measurements of disk and RAM are coming up zeros.  Not sure why this is.
It used to work in Gorax.  More comparative testing between Gorax and
Gophercloud is needed.

However, what is in this commit is sufficient to get the acceptance
tests running again, at least on Rackspace's infrastructure.  If you
attempt to run these tests on HP or other infrastructure, they will
fail for you.