Remove the need for unnecessary args when calling Subscriber.get.

This allows the following code:

s = Subscriber('listid', 'me@test.com').get()

Rather than this, which was previously required:

s = Subscriber('listid', 'me@test.com').get('listid', 'me@test.com')
Remove the need for unnecessary args when calling Administrator.get().
Remove the need for unnecessary args when calling Person.get().
Merge pull request #20 from campaignmonitor/remove-the-need-for-unnecessary-args

Remove the need for unnecessary arguments when calling various methods.