Added virtual method FetchFrames in DebuggerSession to be able to request Frames in bulk and also implemented in SoftDebuggerSession
Removed mirrorThreads caching in SoftDebuggerSession since it's now done inside VirtualMacine
Merge pull request #35 from DavidKarlas/fetchFrames

FetchFrames implementation + removed MirrorThread cache(cached in Mono.Debugger.Soft now)