Merge with shvetsgroup/jquery.multisortable

This version contains all the fixes I've found in different forks of iamvery/jquery.multisortable. It's hard to recall all of them, but you can be sure that they have made sense ;)

In addition to that:
- Improved Shift-selecting so that it works more like in Finder/Explorer.
- The unit tests were updated and fixed.
- Added Travis CI configuration (in last commit I've changed the account to "iamvery", so you should make sure you've enabled Travis integration with your repository).
- Tuned paths.
- Fixed README.