Get/set accessors in interfaces

[beloh]

Porting over this issue from:http://typescript.codeplex.com/workitem/260.

This is a request to add support for get/set accessors in interfaces. This would allow the interface to define the ability of the interface. For example, in the case where the property is read-only, "get" keyword in the interface would indicate that the property is read-only.

[RyanCavanaugh]

Duplicate of #12