Adding methods to math

- Added method_erf and method_erfc and modified method_lgamma in topaz/modules/math.py
- In rubyspec/core/math/lgamma_spec.rb everything passes, in rubyspec/core/math/erf(c)_spec.rb most of the specs pass (in both cases calling the method with something that can't be coerced with Float still doesn't raises a TypeError)
- Also added (or in the case of lgamma modified) tests in tests/modules/test_math.py