Redo commits; update .net and nuget packages

Change to .NET 4.5.1 which requires updating Nuget packages; add
RavenDb
package for new support
redo commits

Replace FileOptimisticStore with DebugOnlyFileStore
Upgrade .net target framework and nuget packages
Upgrade .net target framework and nuget packages
add RavenDb support
Upgrade target .NET version
Upgraded Nuget Packages across projects
Update RavenDB support; Add RavenDB test scenarios

Removed RavenDB folder; put all code including doc class in root folder
where Azure BlobOptimisticDataStore was located.  Added integration test
scenarios for Raven. Because RavenDB client supports caching,
significantly simplified implementation to leverage RavenDB .NET client.
Unused file; replaced by Raven.cs
Start to add async/await support for Raven

Adding async/await support since much of this library depends on I/O.
To be done: modify base classes, unit tests, integration tests for async
support
Add support for async

Add new async interface, async scenarios for integration and/or unit
testing
Move session management as early as possible

Session management now in GetData and WriteData public methods; session
passed in to Create, Read and Update document methods using extension
methods
Separate synchronous and asynchronous support
Merge branch 'Unicorn' of https://github.com/rkayman/SnowMaker

Conflicts:
	SnowMaker/DebugOnlyFileDataStore.cs
	SnowMaker/UniqueIdGenerator.cs
Framework changed to 4.0, nuget update
BlobOptimisticDataStoreAsync added
BlobOptimisticDataStoreAsync
Prefetch logic added