Beginning of pure python py-radix implementation

- Initial goal is to get working against pypy
  - Full compat with existing c extension
  - eventually move away from c extension (except performance critical parts)
No longer allow pypy to fail
Add initial `.search_best()` and `.search_exact()`
Change out Exception types to match C-ext
Cleanup some internals with Node/Prefix
Rename lookup to add
Add .remove(), fix some _prefix bugs, cleanup iterator
Fix masking on prefix creation
Modify testcase to simply check for partial classname
Add pickle back and fix off-by-one bug
Correct a few off-by-one errors and mis-naming
More testing with travis
Fix travis yml file
Fix travis yml file (again)
Make testcases more python2.7/2.6 compat
Python2.6 requires format position
Bump pypy to top to start earlier in travis runs
Cleanup for python3 errors
Finish off python3 compat
A bit of code movement