feat(build): convert .bat files to gulp tasks

- rename makpak.exe to maxthon-packager.exe
- add "dist-zip" task which generates .xpi (Firefox), .oex (Opera),
and .zip (of build/chromium) packages
- add "dist-maxthon" task which uses maxthon-packager.exe to generate
.mxaddon package
- add "dist" task which simply runs "dist-zip" and "dist-maxthon"
- remove .bat files and _zip_packer.py

This closes #46.