jQuery is optional and adapters can be injected

The object is injected so developers can write adapters to use any other library than jQuery.

Inddeed, I've developed a Mootools adapter that you could be interested in: https://github.com/SocialBro/jvectormap-adapter-mootools

The changes I made do not affect to the API so it's safe to merge

**Changes in source code**
By default, it uses jQuery if it's loaded, else it's expected you inject the object later
```
jvm.$ = window['jQuery'] || {};
```

**Changes in the build.sh script**
I've added a new param `--skip-jquery` in order exclude any jQuery file in the concatenation process. By default they are included:
```
./build.sh --skip-jquery
```