Fix malformed token when \n is at the end.

* When the token is read from the dotfile, sometimes it will have
  a newline at the end with which GitHub errors: Bad credentials.
  This does a string replace to remove newlines.
* Test the token when being read from the dotfile before deciding
  everything is ok. Error if authentication fails.
* Show GitHub error message when failing so the user understands
  what's going wrong.
Merge pull request #1 from henrahmagix/fix-bad-tokenfile-error

Token in dotfile breaks this without info when it has a newline at EOF