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.

requirements.txt

@@ -0,0 +1,6 @@
+numpy>=1.8
+matplotlib>=1.3.1
+shapely>=1.3
+simplejson
+rtree
+scipy