Script to clean release directory

You can now do npm run clean:buildDir, ... clean:releaseDir, or just
... clean, which does both.

The release directory is automatically cleaned before packaging
commences.

package.json

@@ -30,6 +30,8 @@
     "build:uncompressed": "gulp buildUncompressed",
     "bump": "npm --no-git-tag-version version 4.$(date +'%Y%m%d').0",
     "clean": "gulp clean",
+    "clean:build": "gulp cleanBuildDir",
+    "clean:release": "gulp cleanReleaseDir",
     "checkin": "gulp checkinBuilt",
     "deployDemos": "gulp deployDemos",
     "format": "git-clang-format",