Reorganize the README, and update its contents to more accurately
reflect the public release.
Remove the incremental "Changes" section.
Rename readme.md to README.md
Rename license.txt to LICENSE
Update the assembler tool to support -h, and make its help look
more consistent with the disassembler.
Change the target and library name to SPIRV-Tools. To better
match the GitHub repo name. Also, it's not SHOUTING.
