Move common functions from converters into module

All 3 adapter converters were sharing a set of common functions so
they've been extracted out into their own module called ExVCR.Converter.

It implements the methods on the converters using the __using__ callback.

They are all overridable.
Filter header values in converter

Headers were unable to be filtered, now they're filtered the same way as
body filters using ExVCR.Filter.filter_sensitive_data.
Merge pull request #3 from BlakeWilliams/header_filter

Refactor Converters, Filter header values.