requirements.txt file: Initial set of technologies I think we will be using

I've added an initial requirements.txt file. Let's use this pull request to discuss any technology choices. 

Note all the sqlalchemy related adds. -migrate adds migration support (we should probably use migrations from day one as our schema will likely change), and the Flask related one seems to be a recommended Flask wrapper for SQLAlchemy.

Additionally, I did not provide version numbers of certain things - take the approach that if I didn't care - I didn't specify version numbers. The Flask related version numbers come from an application @fna earlier developed.  

Thoughts?