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.
Merge pull request #920 from tschaub/point-examples

Cast to number when creating literals from symbolizers where literal properties must be numeric.