Clean up Gruntfile

[mkremins]

Even aside from the confusing-task-names issue described in #97, there's a lot of unnecessary junk in the Gruntfile. Skimming through it, I suspect at least the following can be removed outright:

• The grunt-strip-code plugin and corresponding strip_code config block (we don't actually seem to use these anymore)
• The blah config block (seems to be a renamed, outdated version of the nwjs config block)
• The chdir and dirList sections of the shell config block (unused)
• The grunt-require plugin (we're not using RequireJS anymore)
• The grunt test task (we run the tests in-browser instead)
• The grunt python task (its name doesn't make any sense, and all it does is run the shell:target task)

I'm also not totally clear on how much of the copy-related stuff is actually still in use, or whether the pkg config block actually does anything.

The uglify stuff isn't being used right now, but I can imagine that we might want to start using it again in the foreseeable future, to build a minified ensemble.min.js version of the standalone ensemble.js library file.

[mkremins]

As of the last few cleanup commits, everything that's still in the Gruntfile is actually being used as part of the build process, so I'm going to go ahead and close this issue for now.