First steps with NBT package.

I wrote tests for simple tags, then wrote code to implement them.
Right now I have bytes, ints, and compounds working.  Once I add the
rest, I'll also add support for the example files (test.nbt and
bigtest.nbt) as a proof of concept.
All but TAG_List is done!

Unfortunately, doing TAG_List right will require a little bit of
refactoring of what I've already done, and I think that can wait.
Refactored NBT code in preparation for TAG_List.

The refactoring had the added bonus of improving test coverage to
boot.  I was unable to completely refactor the code due to an
initialization loop error which I would like to fix.  There's also
something wrong with TAG_Byte -- it takes (and returns) []byte instead
of byte which I need to fix soon.  Other than that -- and all the
panics -- this is looking good!
Increased error checks.

Downside is that it's a few more untested lines of code since I'm not
going down every error branch.  Upside is that errors make it all the
way up, which I think is good.

Still haven't done list.
Fully functional!

The code passes all tests but bigtest.nbt, and that one appears to be
correct when read by eye, but for some reason is not coming up
correctly with reflect.DeepEqual.  Need to ask for help here.
Added test files.

These test files are crucial to validating the NBT package.
Minor cleanup in nbt.go

Slightly less repeated code.  Moved declarations out of the loops at
least.
Moved tests from files to byte streams, all tests pass!

Created byte streams from the files, and now the tests pass
repeatedly.  Woo!  On the bright side, I can now put my tests on the
playground if necessary.
Removed no-longer-necessary test files.

Now that the tests have been converted to bytestreams, the raw files
are unnecessary.
Added drone.io badge to README.md.

Now that I have actual executable code, I can use drone.io to test
builds, so I might as well report its build status.
Merge branch 'nbt-package' into develop