+ non-generic interfaces & INCRBYFLOAT methods

Added IClientsPool and IRedisClient without a generic type parameter for
easier use with IoC frameworks when you're just working with strings.

Also added support for INCRBYFLOAT and HINCRBYFLOAT methods released in
Redis 2.6.

Also updated the NuGet.targets file to download NuGet.exe because the
solution wouldn't compile without it.