7.x 2.x webblocks

Several tweaks included with this pull request:

1. Fixed `.gitmodules` reference
2. Merged needed details from `README.md` to `README.txt`
3. Removed superfluous `README.md` and `LICENSE` files
4. Moved `src` to `blocks` as it's not clear what `src` means in this context
5. Updated `Rakefile.rb` to use `blocks` rather than `src` for app sources
6. Updated `Rakefile.rb` to build directly into base repo (as it uses the same `js` and `css` directories as theme)
7. Added the Git submodule file itself (as it wasn't merged earlier)