hardcode Git Bash download to 1.8.4

1.8.5.2 is currently buggy, see
https://github.com/swcarpentry/bc/issues/234#issuecomment-32764692
Merge pull request #260 from swcarpentry/backport-gitbash-fix

hardcode Git Bash download to 1.8.4
Quote $PATH export to account for spaces.

Many people have spaces in their Windows user names, leading to trouble with an unquoted $PATH export.  This file now quotes the $PATH addition accordingly.
Merge pull request #267 from davis68/gh-pages

Quote $PATH export to account for spaces.
Fix 404 for license

The file is `LICENSE.html` and not `license.html`.

W. Trevor King: Cherry-picked from master's f876729.
Corrected location of Windows installer script
Merge pull request #274 from swcarpentry/fix-swc-installer-location

Corrected location of Windows installer script
Initial commit
Pulling in content from main 'bc' repository
Restoring README.md broken by bad shell script