Fix tests command, add bugs section
Merge pull request #207 from nkt/patch-1

Fix tests command, add bugs section
Update CONTRIBUTING.md
Remove 'observed variable is a reference with refcount = 1' check

PDO (tested on PHP 5.5.4) returns objects which do have `isref`
flag set and `refcount` set to 1.
Fix typo in zephir_get_iterator
Fix zephir_hash_get() when in BP_VAR_(R)W mode
Implementing toString shortcut