do not check lon and lat parameters in constructor supporting also metric coordinate systems:

define lon and lat like in OpenLayers (see constructor http://dev.openlayers.org/releases/OpenLayers-2.13.1/doc/apidocs/files/OpenLayers/BaseTypes/LonLat-js.html )
allowing alternative map units (kilometer, meter, mile, foot and inch) additional to degree
(idea from http://dev.openlayers.org/releases/OpenLayers-2.13.1/doc/apidocs/files/OpenLayers/Map-js.html#OpenLayers.Map.units )
handle JTS geomeries that are null or empty (i.e contain no coordinates) correctly (same idea as in https://github.com/lurajon/geogson/commit/877b13c8f4d10b69cd1887e722076c94fea01999 )
handle reading of null geometries and tests

JtsAdapterFactory: handle reading of JTS geomeries that are null (or have been empty)
JtsAdapterFactoryTest: added tests for empty or null JTS Point and MultiPolygon
Merge branch 'customMapUnits'
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
Merge pull request #28 from bkiselka/jtsGeometryFactory

Support for custom JTS GeometryFactory (see Issue #21)