Add more response methods and getBalance method

Add response methods for the response status and the response reason
Add method to get the current credit balance
Add missing docstrings for getHTTPResponseData and getXMLResponseData
Cosmetics
Change default for replaceChars

Set the default setting for replaceChars to True, as in the API
documentation
Add method to set the status report url

Add method setDlrUrl to set the status delivery report url to another
value than on the API settings page
The URL is validated with the standard library urlparse
Minor docstring cosmetics
cosmetics
Merge pull request #4 from jok2/master

Some more API settings / response methods