Removed Router::parseExtensions().

Just use Router::setExtensions() instead. setExtensions() now also takes
a single extension as string for first param.
Rename Router::setExtensions() to Router::parseExtensions().

Also renamed Route::setExtensions() to Route::parseExtensions() for consistency.
Merge pull request #3406 from ADmad/3.0-router-extensions

Removed Router::parseExtensions().