Fix invalid call of the Exception construct method

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.
Performance Improvement (is_null, strlen)

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)
change file_exists to is_file

file_exists check the param against file & dir. is_file check if it is
a file only. Furthermore the performance is much better.
Remove many is_dir, substr calls

Optimize the find_file function and remove many is_dir and substr
calls. Use glob for the recursive selection of files to select folders
directly(without files and „.“ and „..“).
Merge pull request #150 from lochmueller/master

Fix invalid call of the Exception construct method & performance improvments