Blender Git Loki

Drivers: "Scripted Expression" drivers (i.e. PyDrivers) work again

Now it is possible to write a Python Expression using the variable names for driver targets (see mockup from initial commit) to substitute the appropriate values into the expression.

In the __global__ namespace for PyDriver evaluation, the following modules are available:
* __builtins__ - i.e. the builtin Python functions
* bpy - new Python API
* math or m - math module

For example:
Consider a driver with three targets, named: A, B, C
Now, you could write an expression like:
C if A < 5 else B
or even:
2*C if A < 5 or A > 20 else m.PI*B

Of course, you don't have to have three targets, the above was just an example.

TODO:
* Bring back way to load pydrivers.py
* Blender.Noise equivalent would be nice to have

P.S. I hope I haven't made any terrible Python API coding errors here (i.e. mem leaks, etc.)