Add SDK-specific build

"grunt sdk" will produce an SDK-specific build in the build/sdk
directory, as well as an associated wgt package. This
can be installed with "grunt sdk-install".

The SDK build has uncompressed HTML, CSS and JS, but optimised
image files. It also only includes the JS files from dependent
projects (requirejs etc.), rather than the whole project; these
JS files are also unminified.

Note that this uses a newer version of grunt-sdb which ignores
uninstall failures. This means we can dispense with the reinstall
tasks and have generic install tasks which remove any previous
versions first.

Also update HACKING.md to explain the changes.