Content type of "source" and "target" limited to plain text

[achimr]

The v2.0 specification defines the "source" and "target" parameters in the translation request as "Text
is interpreted as plain text.". This under specifies whether the plain text could contain inline markup and in which format the inline markup would be. Such under specification can create issues between different implementers of the API.
An option would be to add a contentType field like in the Translate method of the Microsoft Translator API: http://msdn.microsoft.com/en-us/library/ff512421.aspx