Removed TODO class

- Only used once, not necessary
Better string representations for connections
Removed JSON serialization

- This would be good, but should be implemented better
Removed Constant class; Signals now have values

For a while now, the constant class hasn't actually done anything;
a "constant" was simply a Signal with no Operator that wrote to it.
This finishes the job. It also enables all signals to have
an initial value other than 0, which is necessary for learning.

Now, a Signal can either be created with a shape, which under the hood
creates an array of zeros with the specified shape, or it can be
created with the value, which implicitly specifies the shape.

- Also added a test to test_simulator to makes sure initial values
  are maintained
- Modified a few tests that made Constants
Incorporated nengo.log_to_file into nengo.log

- Also a few other test cleanups
Simplified Signal constructor

- Now, a value must be specified when creating a Signal.
- DotInc and ProdUpdate now have faster make_step functions.
- Also removed model.signals, which wasn't being used for anything.