add script to generate vimball
Merge branch 'master' of https://github.com/jeromebaum/simplenote.vim

Adds script to generate a vimball

References #53
Changes to suit merging in vimball script from @jeromebaum

See commit 2796732f7ce671c225c91abccbff47f7a10b22fa

- Needed to tweak the vimball.txt script as a result of changes from #57
- Add note about installing from a vimball to the README
- Add executable bit to mk_vimball.sh
- Source the mk_vimball.sh from generate_release.sh

Closes #53