Legacy support

Re factoring the package related files to be more generic.

The code was xpk specific. Since we need to support .wgt
formats also , we need to re factor the related files.

As of now the skeleton for .wgt package format is provided.
AFAIK no pre processing should be required as in xpk , like signature checking,
header size, etc. So we can directly now proceed to parsing the xml. 
But need to have second look though. 

@cmarcelo : regarding your question : "I'm inclined to say we should hide the extraction logic inside Package (could still be implemented as a separated class) and expose in Package an Extract() method. What do you think?"

I dont have very strong opinions on that. I can try that in a follow up patch.

please have a look :
@cmarcelo , @pozdnyakov , @shaochangbin , @ds-hwang