Add new Organization management methods and tweak caching.

[phpdreams]

I needed the ability to retrieve just the admins from an Organization as well as get a single member's status within an Organization.

This can be done via:
GitHub::organization()->members()->all($orgName, null, 'all', 'admin')
and
GitHub::organization()->members()->member($orgName)

I also wanted to be able to add/invite a user/member to an Organization. In testing this, I found that caching the PUT request to GitHub resulted in ResponseExceptions as the API would return a 421 response to the request check. The most logical fix was to restrict caching to GET requests only as the other request types all should affect changes at GitHub and therefore render the idea of caching for them moot.

[GrahamCampbell]

Or HEAD too.

[GrahamCampbell]

if (in_array($httpMethod, array('GET', 'HEAD'), true))

[phpdreams]

Good idea Graham, I've added this to the request. Thanks!

[gquemener]

Indeed caching should only be performed on safe http request (ie: GET, OPTIONS and HEAD).

Not sure why it wasn't the case before...

[docteurklein]

We should leverage the cache mechanism that guzzle offers now.
The current one was written before guzzle had it. (AFAIR).