Added support for a custom GeometryFactory (see Issue #21)

specify the GeometryFactory optionally in the constructor of JtsAdapterFactory and pass it on to the JtsGeometryAdapter. AbstractJtsCodec got an abstract constructor that passes the geometryFactory on for further usage and is called by every codec.

Tests also added
Revert "Merge branch 'customMapUnits'" so this branch has only changes necessary for a custom JTS geometryFactory

This reverts commit 2e4cfd6699d4393fbe49b9ebaae442eb01257a64, reversing
changes made to 77854262681f9e1b085ebc1bbe3a9179c2c1fe5f.

branch customMapUnits and nullJTSgeometry reverted
Conflicts in jts/src/test/java/com/github/filosganga/geogson/jts/JtsAdapterFactoryTest.java resolved
Revert "Revert "Merge branch 'customMapUnits'" so this branch has only changes necessary for a custom JTS geometryFactory"

This reverts commit aea2d6c0a640becf1a37ba98b7d211beb66bbeae.