Portfolios can now be filtered with [rescue_portfolio filter=MY_SLUG]

The filter widget (.filter_wrap) is now only rendered, when no filter is set.

If one is set, the `WP_Query` is extended with [tax_query parameters](http://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters).

Extending this to support multiple terms should be as easy as making `$filter_term_slug` an array. But I haven't tested that.

This very pull request was tested locally with PHP 5.5.9 and on a hoster's PHP 5.4.30.