Fixes #244

- GetOrgMembership updated to accept user parameter (BREAKING CHANGE)
- EditOrgMembership updated to accept user parameter (BREAKING CHANGE)
- Added new RemoveOrgMembership method

Ref #187

- AddTeamMembership and ListTeamMembers both take new Option structs to
  specify role.  (BREAKING CHANGE).
- Add Role to ListMembersOptions
- update Permission and Role docs

Ref #187

Memberships API is now official.

https://developer.github.com/changes/2014-09-16-finalizing-the-organization-and-team-membership-apis/

Fixes #192

Latest blog post:
https://developer.github.com/changes/2014-08-28-accepting-organization-invitations-from-the-api/

- TeamMembership has been renamed to Membership, since it now covers
  organization membership as well
- Status field renamed to State
- added new methods: ListOrgMemberships, GetOrgMembership,
  EditOrgMembership

This is a BREAKING CHANGE due to the change in the ListMembers method
signature.

this change also includes some additional minor code cleanup

fixes #58

in the case of org members, combine ListPublicMembers() into
ListMembers(). fixes #8

The truth is, this change makes me sad.  It's a breaking change
for all clients and it adds more complexity to the library surface.  In
most cases, clients will simply drop the Response object on the floor
(which is actually all the library itself was doing before this
change... now we're just pushing that off to the client).

Initially, the Response object will be primarily of interest for
functions that return paginated result sets, since the Response.NextPage
field is the only way to know for sure if there are more pages that
should be fetched.  And this is really the cleanest way to get at that
data, so in that respect this change isn't so bad.

It's also worth noting that returning the raw Response object makes a
lot more since in a GitHub library than it may in others, given how
GitHub makes liberal (read: proper) use of HTTP request and response
headers.  Other APIs, like Google's various APIs for example, tend to
push things like pagination links into the response body.  While this is
certainly less of a purist view in terms of REST, it does make the lives
of client developers a lot easier, since then the response body contains
everything you need to know.  But whatever; this is how GitHub rolls, so
we'll roll right along with them.  (Somewhat ironically we are ignoring
the RESTful links in the GitHub response bodies, since we're actually
calling the API in an RPC style and don't do anything with those links.)

We still don't have an easy way to set arbitrary request headers, but
that's a problem for another day.

Fixes #22