Initial IDistributedCache abstraction and implementations.

This includes:
- The IDistributedCache abstraction
- An implementation using MemoryCache
- An implementation using Redis.

This does not include
- Any serialization helpers
- Any concurrency controls (optimistic, locking, etc.).

Notes:
- Resis does not have any notion of sliding expiration, only absolute expiration. It does allow a single entry to contain multiple key-value pairs of data. We use this to store the absolute and sliding expiration values with the data and retrieve them later when we want to re-calculate the expiration.
- Some redis operations are best implemented via LUA script sent to the server. I've done the critical ones that way so far, but there are a few more (like refresh) that could be optimized via script.
Are you suggesting the sliding expiration be modifiable on every access? That seams overkill. Get will automatically handle the sliding expiration for a hit.