Account for null or undefined geometry

The change in #2098 made it so a feature's geometry could be undefined.  This is consistent with the return type for the getGeometry method.  Where calling code needs to ensure that it has a geometry instance, it can use instanceof, goog.isDefAndNotNull(), or test for a truthy value.
Merge pull request #2107 from tschaub/null-or-undef

Account for null or undefined geometry.
Dispatch change on feature id change
Provide a method for retrieving features by id
Disallow adding the same feature twice
Merge pull request #2087 from tschaub/id-index

Add method for retrieving features by id.
Apidoc readme: minor change

so comments rendered correctly
Merge pull request #2110 from probins/patch-1

Apidoc readme: minor change.