Converted the rest of the tests to pytest

- unittest is now completely gone, except for in
  test_ast_conversion, which I will fix in my next PR
Fixed AST converter

- now works with updated node function signatures
  (getting both t and x as inputs)
- fixed up AST tests to work with new nengo
- fixed a minor bug related to signal shapes in probes
Accommodate changes to op attribute names in Nengo
Got all AST tests working
Scan Nengo directory to select all tests.

- test_sim_npy and test_sim_ocl now run all relevant Nengo tests
- fixed warnings in test_clra_nonlinearities
  by making n_neurons integer
- made logging messages not show up all the time by setting
  default logger to none
Updating test-finding functions to match Nengo.
Merge pull request #57 from ctn-waterloo/ast-fixes

Fixed AST converter, updated to py.test