RedisSessionProvider

Add DatabaseIndex to SeRedisServerConfig. and key Compatible.

add database index to connection parameters.

Add target="" to the readme.txt file in nuspec so that when the package is installed via nuget the file is not added to the installing solution but the contents are shown in visual studio. Removed web.config.install.xdt auto install for the sessionstate tag, since its a little invasive.

Modified RedisSessionAccessor's Session property to be a subclass of HttpSessionStateBase

Added RedisSessionAccessor, made some internal tweaks to RedisSessionStateStoreProvider to support code reuse among them (though that probably should get refactored into a third utility class), added session timeout and cookiename to the RedisSessionConfig class