Fix writing of unnamed NBT tags

Fixes an error that popped up in BO3Tools, but was caused by Terrain
Control. This bug existed before Minecraft 1.7, but Minecraft 1.7 uses
unnamed tags on many more places.

Also made the class a little easier to work with: the
findTagByName(name) and findNextTagByName(name,previous) methods are
replaced by getTag(name) (for compound tags) and values() (for list and
compound tags). Unlike findTagByName(name), getTag(name) doesn't search
recursively in all nested lists and compounds.