Add an SDK-specific grunt build

Add grunt targets to make and install an SDK-tailored version of the app:

* grunt sdk does an SDK build, copying the required files to build/sdk and creating a wgt package (unsigned) from those files. The build/sdk directory can be used as example code in an SDK environment, as it contains no redundant Chrome-specific files (like manifest.json and _locales), no files irrelevant to SDK users (like HACKING.md), no build infrastructure (no bower or npm configuration), and no minified files (other than the images).
* grunt sdk-install will run the sdk task (see above) then deploy the SDK version of the app as a wgt package to an attached device (requires sdb).

Note that the package produced by the sdk target will not perform as well as a package produced with the wgt target, but the apps in both packages are otherwise equivalent.

Note that I also fixed a bug I spotted while testing the SDK build. I didn't take the time to produce a separate PR for it, but could if necessary.

And corrected an error in the README.