Add ApiError and TwitterError types to allow error matching
Remove unnecessary logging
Add an example and a test for Twitter error handling

* This tests an error that originates with Twitter, as opposed to other
errors
Move error definitions to dedicated file (error.go)
Fix dangling comment left over from the previous commit.
Update package documentation information for godoc
Cursors should be int64, not int
Implement GetFollowersList, which returns a list of followers (cursor)
Print tweets on separate lines in example code in README
Export Sender and Sender.Screen_name in directmessage struct
Merge remote-tracking branch 'upstream/master'
added user methods, made users return values intead of pointers, merged in error handling