immutable timings
Corrected import to graph algorithm
Tail recursion on retrace
Wrapped up priority queue and map
Added xunit runners
Partial bidirectional fix
Merge remote-tracking branch 'origin/immutable'

Conflicts:
	TestsInFSharp/Engine/Searcher.fs
	TestsInFSharp/TestsInFSharp.sln
	TestsInFSharp/TestsInFSharp/OsmImportTest.fs