Simpler read api

I added some methods for easier traversal of a NBT tree. Its focus lies on reading compound and list structures with primitive values.

An excerpt of the new ReadWriteTest to see how it works:

```
TagCompound tag = (TagCompound)nbtInputStream.readTag();
Assert.assertEquals(4.2d, tag.getDouble("doubleTag"), 0.1);
Assert.assertArrayEquals(new int[] { 21, 42, 84 }, tag.getIntegerArray("integerArrayTag"));
Assert.assertArrayEquals(new String[] { "Hello", "World!" }, tag.getStringArray("listTag"));
TagCompound inner = tag.getCompound("compoundTag");
Assert.assertEquals("Bananrama", inner.getString("name"));
```