Improve

#15, #17, #18

Basically:
- added a `force` option to override links
- symlinks over directories are possible
- takes an array of destinations, a function, a Vinyl instance etc.
- keeps the same API as before

This PR is ment to improve gulp-symlink according to https://github.com/soyuka/gulp-sym#why.

What's left: 
- add those informations to the readme
- add a #19 fix to stream links instead of source files

Thoughts? I've try to keep this as simple as possible especially with the unit test part but it wasn't easy (:.