Sharing methods

[spatten]

This adds the following methods to Client:

• list_shared_folders
• continue_list_shared_folders
• share_folder
• add_folder_member
• mount_folder
• list_folder_members
• relinquish_folder_membership
• transfer_folder

There are tests for everything, and an integration test that goes through the following workflow:

• Create a folder
• Make it shared
• Share the folder with a second account
• Accept the share request as the second account
• Upload a file as the second account and make sure it shows up in the first account's Dropbox
• Transfer owner ship from the first to second account
• First account relinquishes ownership

This requires a second Dropbox access token passed in as DROPBOX_SDK_ACCESS_TOKEN_2.

I think that's it. This is a pretty big change, but I tried my best to maintain the same style as in the existing code. Hopefully it's all good.

Thanks,

Scott