Introduce Grunt build targets

This change introduces the concept of build targets to Grunt, these targets are defined in `target-config.js` and have different sets of source and test files.
The current build targets available are:
 - minifill
 - minifill-inline-style
 - minifill-inline-style-methods
 - polymerfill

*Changed grunt commands*
`grunt`
The default grunt command now also runs `grunt gendev` and `grunt gentest` in addition to `grunt uglify` and `grunt gjslint`.

`grunt uglify`
This now builds uglified web-animation-*.min.js files for each of the build targets.

*New grunt commands*
`grunt gendev`
Generates web-animation-*.dev.js files for each of the build targets intended for debugging.

`grunt gentest`
Generates test/runner-*.html files for each of the build targets intended for running unit tests specific to the build target.

`grunt clean`
Removes all generated files.

`grunt <build target>`
Runs `uglify`, `gendev`, `gentest` and `gjslint` for just the specified build target.