Stop inflecting API method parameters on each call

[petedmarsh]

Previously, every parameter of an API call was converted to camel case before being serialized to JSON - even if it had been converted before. This added unnecessary overhead to every API call.This avoids these conversions by manually constructing dicts with the correct keys from the parameters passed to the method.

This is a little less pretty but is the most straight forward way of achieving this.

The code passes flake8 checks, but if there are any formatting changes you would like I will happily make them.

[petedmarsh]

The flake8 errors are unrelated to the changes in this PR, https://github.com/jmcarp/betfair.py/pull/54 will fix them.