[Axis2] renaming core as engine

[Damitha Kumarage]

Hi,
Currently we have followed axis2 java project folder structure so that 
we have
                    modules
                         |
|-----------------------------------------------
                           |
                         core
         ----------------|------------------------
        |                    |                              |
 context           description               engine
I suggest that we replace  path beginning from core as engine. Then we 
have all core related files in engine.
This is similar to axis c++ folder structure where we have common, soap, 
wsdl, engine , xml, util etc
regards
damitha