Ignore extra dimensions and pad with NaN for missing dimensions
Add a concat function for concatenating strings
Call concat instead of using math op in FE expression parsing
Cast to numbers for literals

There are times when we parse from XML without a schema (e.g. KML).  In these cases, features attributes will always be strings.  We can cast to number when creating literals from symbolizers and then assert `!isNaN` instead of asserting that they are numbers before.
Add KML earthquake example
Missing test dependency
Leaving the + operator as a convenience for concatenating strings

When people need to serialize in a format that differentiates string concatenation from addition, they can use the new `concat` function in an expression.  But I don't think we need to throw if people use `+` with strings.
Add externs for GeoJSON crs member
Read crs from top-level object when parsing as features
Merge pull request #913 from tschaub/dimension

Allow for vector data with unknown or inconsistent dimension.
Remove unnecessary cast and save 15 bytes using goog.isDef
Merge pull request #921 from tschaub/geojson-crs

Parse top-level crs member when reading features from GeoJSON.
Merge pull request #920 from tschaub/point-examples

Cast to number when creating literals from symbolizers where literal properties must be numeric.
Export ol.source.Source#getExtent
Merge pull request #924 from elemoine/getextent

Export ol.source.Source.prototype.getExtent
Accept null crs
Add ol.CollectionEvent#getElement
Use ol.CollectionEvent#getElement in LayerGroup
Export ol.CollectionEvent#getElement
Use ol.Collection#getElement in tests