`Plug.Conn.collect_body/3`

Move `Plug.Parsers.URLENCODED.read_body/2` to `Plug.Conn.collect_body/3`
Update parsers API
Use `Plug.Conn.collect_body/3` for URLENCODED
Use a single line for these simple return values
Test additions and updates
Add documentation
Merge pull request #41 from scrogson/body_stream

Allow easier access to the request body