switch account capabilities

Added account switching capabilities - this feature was built in DEComposeTextView but was not exposed to any later above that.

Now you can define some additional methods on your `REComposeViewController` delegate (`defaultAccountName` and `displayAccountsPicker:`) to provide the default account name and the hooks for your delegate to display and let the user pick the appropriate accounts.

Included in this pull request is an example using the user's defined Twitter accounts in the system.

Please let me know if there's anything I can fix, help with, or change to get this pull request accepted.