Start testing things, first with RGB555->RGB888 conversion.

You need Google Test for this. Despite recommendations to the contrary,
I'm not a fan of incorporating third-party code when I don't need to
modify it heavily or compile it with a weird and essential collection of
flags.
Add tests for RGB -> YCbCr conversion.

Currently, the full-white (#FFFFFF) test added here fails, careless use
of floating point is likely to blame :) .
Add test for TGA file generation.

Not a lot to test right now, so it's only one test.
Add more tests for things.

Specifically offset subtraction, game name, and LZSS decompression.
Update readme

This is really needed :)