Documented the fact that NbtReader constructors throw an ArgumentException if given stream is non-readable.
Fixed being able to add a NbtList to itself. Note that recursive inheritance is still possible with longer loops -- it would be too expensive to traverse the whole tag hierarchy on every Add() to check for loops.
Fixed another bug in the way NbtReader handles nested lists. This was partially addressed in 0.5.1, and now fixed in all cases.
Improved error checking in NbtList's Add method and indexer.
Updated unit tests to cover recent changes/bugfixes.