Add methods for accepting and cancelling friend invites

[JonnyWong16]

Description

• Adds a new object MyPlexInvite which is separate from MyPlexUser.
• Adds methods to retrieve sent and received invites MyPlexAccount.pendingInvites() and MyPlexAccount.pendingInvite(user) which return MyPlexInvite objects.
• Adds methods to accept a friend invite MyPlexAccount.acceptInvite(user) and cancel a friend invite MyPlexAccount.cancelInvite(user).

Breaking change:

• Pending invites are no longer listed along with MyPlexAccount.users(). Use MyPlexAccount.invites() instead.

Fixes #767

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable