Update struct definition for DirectMessage
Add Place struct definition
Update several struct definitions

* Retweet has been replaced with Tweet, as there is no difference
* Tweet now includes all fields returned by the API
* TwitterEntities has been renamed to Entities
* TwitterUser fields are no longer pointers (these are no longer
necessary after Go1.1 was released)
* A number of int/int64 fields were previously classified incorrectly
as float64 or *float64; these have been fixed