Merge 3 minor fixes from forcetk library (in developerforce) to entity framework's forcetk.
1. Fix retrieve method's logic
2. Update version to v27.0
3. Dont send contentType for GET or DELETE as some libraries like in Node.js Express BodyParser middleware breaks.
Merge pull request #104 from rajaraodv/unstable20

Merge 3 minor fixes from forcetk library (in developerforce) to entity framework's forcetk