Add requirements.txt for pip.
Add requirements.txt for pip.

This simplifies installation on OSX by installing requirements with a single
```pip install -r requirements.txt```
Library minimum versions taken from http://flatcam.org/manual/installation.html#requirements
Where versions have not been specified, requirements.txt reflects this.

Denvi/FlatCAM,Denvi/FlatCAM,Denvi/FlatCAM,Denvi/FlatCAM