Feature: Naming dynamic helper objects

[chrylis]

The ability to use registerDynamicHelper to map class methods onto helpers is extremely helpful, but currently the method name is used with no ability to customize, and all helper methods go into a global namespace. It would be useful to be able to specify a prefix like this:

hbs.registerHelpers("$math.", java.util.Math.class);

and then say

{{ $math.sin(degrees) }}