Add a "minimum-stability" feature

Hi,

I've added this feature so it is now possible to fetch packages with a minimum given stability level by adding the following option to the json config file:

...
"minimum-stability": "stable"
...

just like in composer (see http://getcomposer.org/doc/04-schema.md#minimum-stability for possible values).

This level was previously hard-coded as "dev".

PS: please tell me if I did this the wrong way as this is my very first pull request :-)