Check default/override members via 'IsOverrideOrExplicitMember'

Close #208.

This PR exposes a few symbols related to default/override members that are previously hidden in FCS. Now there exist symbols for `abstract` and `override` members to provide enough information for generating signatures similar to those from tooltips.

![image](https://cloud.githubusercontent.com/assets/941060/4115421/b3b6a858-3277-11e4-8cc8-9040cec55431.png)