Adds support to store third party libraries out of the src directory tree.

I try to keep my javascript code separate from third party libraries. Something like

    src
        main/js
           myJavascript.js
            ...
        webapp/js/lib
            jquery.js
            ....
        test/js
             myJavascriptSpec.js

So I need for those libraries to be available during unit testing. I've modified the pluging to enable serving the files in the path *jsLibSrcDir* under the *libDirectoryName* path.

The above directory layout will match this configuration:

``` xml
        <configuration>
          <jsSrcDir>src/main/js</jsSrcDir>
          <jsTestSrcDir>src/test/js</jsTestSrcDir>
          <jsLibSrcDir>src/main/js/lib</jsLibSrcDir>
          <libDirectoryName>js/lib</libDirectoryName>
```