Add convenience methods for group memberships

[rdegges]

We should add Account.removeGroup() and Group.removeAccount() stuff to make things nicer for users =)

[luyizhao]

+1. Very useful if you're using webhooks to manage account groups.

[robertjd]

Agreed! Hey @luyizhao, I'd love to hear how you're using webhooks with Stormpath!

[luyizhao]

Mainly Stripe events which dictate the groups users should live in. Updating customData upon events is a decent workaround, although sometimes it's still better to roll your own removeGroup() functions to bypass caching with customData :) ... (Sidenote, interested in seeing how future cache-handling for groups develops as referenced here!)

[robertjd]

I agree that we should add these methods, but what should the inputs be to these functions? An already-constructed Account or Group object? That makes the most sense, but what if I have just an href or name? We have to define these different options