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'