Tomorrow you go in, and tell them that you want to get paid for the work you've done so far, and you want to get paid now. If they start with any excuses, you simply get up and leave. Lesson learned (hopefully). If they agree, you get paid, and then establish your pay rate going forward. I recommend charging a market rate (whatever a good software engineer will charge you in your country for similar type contract work). Oh, and make sure you have a good looking portfolio (or github account).