Upped .NET framework requirements from 2.0 to 3.5
Minor code cleanup in NbtBinaryWriter/NbtReader/NbtReaderNode
Initial commit of NbtWriter class, for high-performance writing of NBT data to streams.
Minor optimization in NbtBinaryReader and NbtBinaryWriter
Sped up NbtWriter significantly by removing duplicate-tag-name checks. I'm shifting that responsibility to the API user.