Added syntax sugar for throwing exception

Now we are able to throw strings, chars, doubles and integers
Fixed warning in isset
Missing separation in zephir_concat_self
Merge pull request #564 from nkt/feature/expression-exceptions

Added syntax sugar for throwing exception
Taking advantage of RETVAL_ZVAL_FAST in PHP 5.6
Merge branch 'master' of https://github.com/phalcon/zephir
Taking advantage of RETVAL_ZVAL_FAST in PHP 5.6 (2)
Try to migrate to extension usage