Enhancement: handy methods in Context.java

[jabiercoding]

It will be nice if Context have the methods, getEntity(String id) and getAttribute(String id). This can be done with two maps. However, this means that the API user will need to use always addEntity and addAttribute, instead of getEntities().add

[jrfaller]

This is a very good idea!

I think that it would therefore be mandatory to have getId in both Attribute and Entity classes. I will try to do it.