Adds Grain.Cache project

Allows *.Cache in gitignore.
Adds Grain.Cache - a cacheprovider interfaces that are not implemented
anywhere yet.
Adds Couchbase Provider

and a stub for MemoryCacheProvider.
Adds comments to the interfaces
Adds CacheProfileManager
Makes the Get<T> more flexible

The .NET Couchbase client GetJson method isn't as reliable as Grain's
own FromJson.
Adds overloads to get many records in single call
remove unnecessary couchbaseprovider overload
IEnumerable Gets and Couch Extensions

Adds new overloads for IEnumerable Gets.
Adds Couchbase View extensions.
ExpiresIn is now nullable for Cache Config
Fills gap that allowed diff keys across commands

Exists and Remove in the CouchbaseProvider had overloads that didn't use
the GetGroupKey.
Merge pull request #5 from losandes/cache-provider

Cache provider improvements