Add support for Listing Servers.

Up to now, all you could do is authenticate with Gophercloud.  Now, you
can list your servers too!

This is a pretty straight-ahead port of logic from Gorax to Gophercloud.

After you gain access to the ComputeApi, you simply invoke
ListServers().  This returns a slice of Server structures, one each for
the servers your account has access to.  See the 02-list-servers.go
acceptance test.

Note that no unit-level tests exist for this functionality.  Writing a
unit test for this code would only serve to test the compiler's ability
to emit bug-free machine code.  (Observe ListServers() has no control
flow points, and all its dependencies already have their own unit
tests.)

Being that this is a straight-ahead port from Gorax, some things remain
to be done:

1) Provide an API for accessing Rackspace-specific fields. (#34)
2) Assuming (1) is successful, remove Rack-specific fields from Server
   and other structures of interest. (#34)
3) Rename ComputeApi to something more descriptive -- like ServerApi().
   (#33)
4) Implement token re-authentication logic.  (#12)