Added Nested Response Writing Methods

[gerardtoconnor]

Added 3 common extension methods to context for writing json, xml & text so that users can do nested writes instead of using pipeline functions. #113

[gerardtoconnor]

seems to be an issue with travis, config error with powershell

$ powershell ./build.ps1 -Release
powershell: command not found
The command "powershell ./build.ps1 -Release" exited with 127.

[dustinmoris]

Thanks for this PR. I'll fix the Powershell issue now according to this solution. One thing which I will probably rename is WriteXML to WriteXml as I've already started to write Xml with a lower m and l in other places and because I think this matches more to what MS has also done in the standard lib (e.g. XmlWriter), otherwise looks great!