_id must go first + test for updating accross indexes
Merge branch 'idfirst' into geonear

Conflicts:
	src/main/java/com/foursquare/fongo/impl/Util.java
	src/main/java/com/foursquare/fongo/impl/index/IndexAbstract.java
	src/test/java/com/foursquare/fongo/FongoIndexTest.java
	src/test/java/com/foursquare/fongo/FongoTest.java
respect insert order in GeoIndex