Introduce UrlGeneratorHelper

Useful to generate URLs based on configured relations

Should fix #91
Introduce ExpressionFunctionInterface

to extend the ExpressionLanguage with custom functions.

* Add 'link' function that relies on the LinkHelper class
* Add tests

Should fix #83
Add more tests
[doc] add doc for helpers
Merge pull request #96 from willdurand/url-generator-helper

Introduce LinkHelper