Fix invalid call of the Exception construct method & performance improvments

The Exception is sometimes called without the second parameter. This
trigger warnings in PHP. I change the second parameter to optional. The
check if the $object is a object is already in the code.

I use phpsass in a complex env with many sass files. The init
performance is bad (after the generation the css will be cached). So I
start same improvements… This patch will reduce about 1.1 Million
is_null and 500.000 strlen calls that are useless (strlen) or should
replace with PHP language constructs (is_null)
![bildschirmfoto 2014-04-02 um 21 11 10](https://cloud.githubusercontent.com/assets/3907126/2595745/a3372616-ba9e-11e3-9c43-4807e92264e1.png)
![bildschirmfoto 2014-04-02 um 21 18 06](https://cloud.githubusercontent.com/assets/3907126/2595746/a338b7c4-ba9e-11e3-9df4-fd0bf99a1e27.png)