Implement Access.FirstEndpointUrlByCriteria

The FirstEndpointUrlByCriteria method is a key enabler for writing API
provider interface constructors.  A simple, albeit incomplete, use-case
for Cloud Servers API demonstrates how it's used internally.

See squashed commit history below for more detailed rationale behind the
API design.

Squashed commit of the following:

commit 625c31f754dcdcd2d348cf4cf5499a03ba6b2de1
Author: Samuel A. Falvo II <sam.falvo@rackspace.com>
Date:   Tue Jul 2 18:21:36 2013 -0700

    Fix service name typo

commit c6abcbe20bfe31a8c9399e78c186dca64d050140
Author: Samuel A. Falvo II <sam.falvo@rackspace.com>
Date:   Tue Jul 2 18:15:41 2013 -0700

    Added decision logic to FFEBC function.

commit bccf7178464c5071a81d63ef16fd20d7a241146f
Author: Samuel A. Falvo II <sam.falvo@rackspace.com>
Date:   Tue Jul 2 17:18:14 2013 -0700

    Added ListServers and its dependencies.

    In order to list servers, we need access to a cloud server API.  This is
    the job of the ComputeApi() function.

    ComputeApi(), in turn, tries hard not to contrain the user in choosing
    an endpoint, while still offering an interface optimized for the common
    case of using an existing service provider's endpoints.  Otherwise, the
    user will end up having to use nested functions and bizarre predicate
    sequences like this:

      func(ce *CatalogEntry, ee *EntryEndpoint) bool {
        if ce != nil {
          return ce.Name == "cloudComputeOpenStack"
        }
        if ee != nil {
          return ee.Region == "DFW" && ee.VersionId == "2"
        }
        return false
      }

    The current interface just encapsulates this kind of logic into a simple
    structure, taking 66% fewer lines, and zero chance for error:

      ApiCriteria{
        Name: "cloudComputeOpenStack",
        Region: "DFW",
        VersionId: "2",
      }

    FindFirstEndpointByConstraint() is invoked (via
    AccessProvider.FirstEndpointUrlByConstraint()) to actually look for a
    matching endpoint in the provider's service catalog.  This interprets
    the ApiCriteria structure settings, except for UrlChoice.  If it finds a
    candidate endpoint, the user may select public or private endpoints via
    the ApiCriteria.UrlChoice setting (which the
    FirstEndpointUrlByCriteria() function interprets). If nothing is found,
    an ErrEndpoint error will be returned to the caller.  Of course, this
    being a brand new implementation, it just returns the default of
    "nothing found" for all queries anyway.

    If not specified, a criteria's UrlChoice defaults to PublicURL.

commit 9549f0b30e0736962dad55f3f38f88124e076fb9
Author: Samuel A. Falvo II <sam.falvo@rackspace.com>
Date:   Tue Jul 2 17:10:14 2013 -0700

    Removed VIM temp swap file

commit 8e00ad5ac3466cbec3c539e8b21bea6d23ab37f7
Author: Samuel A. Falvo II <sam.falvo@rackspace.com>
Date:   Tue Jul 2 16:20:22 2013 -0700

    Add ApiCriteria to API

commit 6f3b41929a496c6a0734221bf12ef27035b71e39
Author: Samuel A. Falvo II <sam.falvo@rackspace.com>
Date:   Tue Jul 2 16:18:49 2013 -0700

    Add acceptance test for list servers