Fix bug with js tree drag and drop not working if user cursor overlaps the marker line.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #63
| License       | MIT
| Doc PR        | 

The only problem with this is it might not fix it for all browsers (the css pointer-events property is not completely supported). This is primarily an issue with IE < 11

http://caniuse.com/pointer-events