Refactor Converters, Filter header values.

In one of my applications I'm using ExVCR but need to parse out the value of an API key in the headers as well as the body. 

This PR removes all the common code from the 3 adapter converters and places it into a new module called  `ExVCR.Converter`. All of the methods are overridable.

Finally, `ExVCR.Filter.filter_sensitive_data` is called on all of the header values.