Adding tests for cache
Changed CacheItemModel properties to nullable.
Corresponding change in Cache.cs

Moving assignment of EffectivePercentagePhysicalMemoryLimit and EffectivePrivateBytesLimit outside loop in Cache.cs
Expanded try/catch in TryGetCacheItemModel.

Added tests to CacheShould.cs.
Merge pull request #675 from bryanjhogan/cacheTab

Improvements to the Cache tab