Refactor harvesters

[heikki]

I think there is room for improvement in the Java code of the harvesters, for the following reasons:

• many lines of code are repeated in almost all harvesters
• the harvesters have basically either one of two basic structures: with the "aligment" in a separate class, or not
• the init() method could be an abstract method in AbstractHarvester, improving contract of harvester classes and removing need for reflection when registering them

These improvements will facilitate making changes.

[heikki]

This is related to ticket #349 "Improve harvesting configuration". Jose has started some work for #349 already.

[heikki]

See also ​http://trac.osgeo.org/geonetwork/wiki/refactoring_harvesters