added method to set parser-options

I've created the new method ``setOptions()``, to set default-options, used in the ``parse()``-method.

Sample-use:
```
$lc = new \lessc();
$lc->setOptions(array('cache_method' => false);
$content = $lc->parse($lessContent);
```
Reason for the implementation: You can pass your options to the Parser itself (``\Less_Parser($options)`` - as I've described in #184 ), but if you use the ``\lessc()`` class directly, you can only use the default-options from the parser.