Concurrency issue for the parser collection in StackTraceParser

By using Approvals in a MbUnit executing tests in parallel I discovered an issue in StackTraceParser that the initialization of the parsers collection is not thread-safe. I fixed the issue by using a static constructor, this way we ensure that the collection initialization is thread-safe.

![image](https://f.cloud.github.com/assets/2493565/1684262/f3912a4a-5dc8-11e3-93a3-9e3b179745a0.png)

I've also added a unit test.