Force use of keyword arguments in method calls

Method signatures are unstable due to a side effect of the implementation of https://github.com/palantir/conjure-python/pull/14. In order to prevent difficult to debug breaks we should force keyword arguments to be used.

This can be supported out of the box in Python 3:

def fun(*, arg1, arg2):
  pass

But we'll have to get creative for backcompat with Python 2:

def fun(*args, arg1, arg2):
   if args:
     raise ValueError("...")

@ferozco
cc @JacekLach, @jamding

Review the implementation changes from pull request 14 and locate the generator code that emits Python client method signatures. Determine a Python 2-compatible way to reject positional arguments without breaking supported compatibility. Done means generated methods consistently require keyword arguments and tests cover the behavior.