Added txt format support

This format allows you to get a "plain text" value, i.e. a
untouched copy of the response body.

You can also read not-properly formatted data.

It just adds a xml node named 'entry' with the response body
in wrapped in cdata.
added issues url
Added some documentation

1. Updated the supported format list.
2. Added documentation about the API (prepareGateway and exposeData)
make static calls consistent
fixing mix between label and value
2.1.0 release notes
Merge pull request #22 from DeuxHuitHuit/text

Multiple small fixes