Fix issue #61 by adding a disable_grep flag.

This pull request solve the issue #61 when using this bundle on Solaris (or any OS on which the grep version is quite old) by allowing to disable the usage of grep thanks to a simple configuration flag.

Tests are running well on my Debian install:
```
....SS..................

Time: 5 seconds, Memory: 57.25Mb

OK, but incomplete or skipped tests!
Tests: 24, Assertions: 54, Skipped: 2.
```

I guess the two skip tests are due to the tests of findstr (available on Windows only).

Otherwise, I think it would be great if this fix could be backported to the 1.1.* version of the bundle (for Symfony 2.1 usage, which is still maintained for at least a month). @schmittjoh What do you think ?