Redis EVAL and EVALSHA fixes and tests.

Fixed:

* EVAL and EVALSHA need numKeys as first argument after script / SHA otherwise ERR is thrown - http://redis.io/commands/EVAL
* When calling an aliased method with `__call()` only `$args` should be passed as arguments.

Added:

* Tests for `eval()` and `evalSha()`
* Alias from `eval()` to `_eval()` function (like in phpredis, `Redis::eval()` is a valid function)