Improve watch task

I'm finally hitting my stride on this project, and pressing F5 is
finally starting to get old. This commit brings in browsersync and a
bunch of gulp.watches to make workflow smoother.

Due to cbarrick/gulp-run#17, gulp-run does not notify gulp of when the
task completes. To fix this, I reimplemented those tasks in terms of the
standard child_process module. I'm pretty pleased with how the tasks
look now.
Remove gulp-filter (for now)

Since I stopped using main-bower-files, I don't need the filtering right
now, so this dependency can be removed.

Note: in the future, I might compile min in the css task (using less),
in which case it would be useful to have the filtering again. We can
always add that dependency back when it becomes useful.