add a few more helper methods

 - `send_line`: sends a line, so you don't have to remember "Enter"/"C-m" after every line
 - `rename`: helpful for renaming the original window tmuxstart creates
 - `select_layout`: does what it says on the tin. `select_layout editor main-vertical`
update README.rst with new helper methods
Merge pull request #7 from BrianHicks/patch-1

add a few more helper methods