Fix installation via Composer

As you don't have any tagged versions yet, the current command does not work when the global minimum stability of the user is `stable` (which is the default.) So, the only way to get this working for everyone is to force the installation of `dev-master` and install the oauth2 dep first. As soon as you release a stable version, you will be able to simplify the install command.
Merge pull request #20 from fabpot/patch-1

Fix installation via Composer
fixed autoloading when installed globally via Composer
Merge pull request #21 from fabpot/autoload-fix

fixed autoloading when installed globally via Composer
made drush optional
Merge pull request #22 from fabpot/drush-optional

Make dependency on drush optional