Expose Xml documentation

Close #677.

There are two versions of `getXmlDocBySignature`, one uses Visual Studio services and the other uses XDocument and external files.

This PR mistakenly includes the other PR. I will rebase when the other gets merged.
Ha I know, because the real comment in intellisense is
Initializes a new instance of the System.Tuple`2 object

The typename part is missing from the XmlDoc here.

![image](https://cloud.githubusercontent.com/assets/1169339/4468589/62fb9e42-4900-11e4-9eab-d2984328fa6c.png)