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.