perf: Maintain shared XML parser instance per thread
perf: Maintain shared time format instance per thread

  + Expose the time format as base DateFormat rather than the overly specific
SimpleDateFormat, which use might be replaced with more optimized
implementation.
Implement cache on filesystem level. About 20% improvement.
Clean up in SymbolicPage.getChildren()
Cache page data once read. Avoid unnecessary I/O.
Update transparency on breadcrumbs bar
Revert "Implement cache on filesystem level. About 20% improvement."
Performance tests do not prove a good enough increase.

This reverts commit d8753d7f233726687bd507bd7d36466411a5508a.
Remove fitnesse.util.Cache. No use for this class.
Merge pull request #473 from stanio/master

Small performance optimizations