Improve PSR-2 compatibility about static keyword.

From 4.5 of the PSR-2 specification:
http://www.php-fig.org/psr/psr-2/

> When present, the static declaration MUST come after the visibility declaration.
Merge pull request #43 from hkdobrev/psr-2-static-keyword

Improve PSR-2 compatibility about static keyword.
Add example usage of refresh token.

Support for the refresh token grant type is added to the client controllers
and the views. It is presented to the user with ability to renew the
access token via the refresh token.