Feature allow changing caching strategy

[keshavkaul]

Motivation:

1. I'm using Easycaching.Redis as caching provider, and I'm getting the error in CachingReader class when trying to cast a DBNull value into a primitive type like string.
2. I want to change the CachedResult type into a more json/messagepack serializer friendly class.
3. I want to try using DataTable.CreateDataReader as the data reader instead of CachingReader.

I've tried to be backwards compatible.
Request you to accept this PR or provide any comments on improvement.

Thank you for creating this library. I'm open to contribute a small amount if you're okay with it.