Complete Cache/CacheDependency implementation and add additional tests.

[CZEMacLeod]

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Complete Cache/CacheDependency implementation to cover additional scenarios
System.Web.Caching.Cache - Support for dependencies (implemented via ChangeMonitors)
System.Web.Caching.Cache - Add missing Insert method
System.Web.Caching.CacheDependency - Add additional constructor overloads (and correct accessibility of 'empty' constructor)
System.Web.Caching.CacheDependency - Fix issue when start date is not supplied
Add tests for dependent files, and for dependent items
Add tests for cache insert
Fix issue when a CacheDependency has no changeMonitors

Addresses #347 and #350

[twsouthwick]

LGTM! I like it

[CZEMacLeod]

If anyone needs AggregateCacheDependency in the future, I do have that code too.
https://github.com/CZEMacLeod/systemweb-adapters/tree/czemacleod/aggregatecache

[twsouthwick]

If anyone needs AggregateCacheDependency in the future, I do have that code too. https://github.com/CZEMacLeod/systemweb-adapters/tree/czemacleod/aggregatecache

Feel free to open a PR for that - if you've needed it, it's likely others will as well. Thanks for your contributions here @CZEMacLeod !