Update Build Requirements and Instructions

Updates our build requirements based on our new testing environment.

- `Cython` required version is bumped to 0.19 due to relative imports from `_shared`.
- `Matplotlib` and `networkx` have been made optional since our tests now pass without them.
- The min Python version is bumped to 2.6 since that is the lowest we test against.
- The min `Scipy` version is dropped to 0.9 since we are testing against that version on Python 3.2

Dependencies are stored in `requirements.txt` and read into `setup.py`.  In the future, changes to versions will have to be made in `requirements.txt` and `DEPENDS.txt` only.

This should work, since we ship the .c files on PyPi. But lets wait for @stefanv.