Ashish/Pooja - adding support to register functions in given namespace.

At present jade4j doesn't allow us to use functions in jade.
For example
```
p= math:rnd(1.22)
```
cannot be used in jade4j, even though underlying JexlEngine supports this by registering objects in the given namespace - math in the above example.

We have added a test in CompilerTest for the same, along with allowing the support to register objects.

As part of this pull request, now this can be done like:
```
JadeJexlEngine.getInstance().setFunctions(funcs);

where funcs is HashMap<String, Object>
```