Change content type from application/xml to text/plain when creating projects

As per the TeamCity REST API documentation, to create a project you
need to POST the plain text name, so the content type needs to be
"text/plain" and not "application/xml".
Added test for project creation
The "accept" header needs to be specified so we get application/json and not application/xml in return
Merge pull request #72 from okb/create-project

Change content type from application/xml to text/plain when creating projects