Export float/double values in decimal notation. Fixes #1781.

This aligns the representation of double values with the format used
in the browser, as well as producing a notation that can be feed back
to Cypher.

Note that the shell still prints double values in scientific notation, so there's some inconsistency left, assuming decimal notation is the way to go (currently the only form accepted by Cypher). From what I gathered, the shell uses *org/neo4j/cypher/internal/compiler/v2_0/commands/expressions/StringFunctions.scala:69* to format a double value, which calls _Object.toString()_ which uses the scientific notation. However, if that is to be changed I'd like someone else to take over as I'm just getting into Scala and I can't really judge the ramifications such a change in the heart of the Cypher compiler would have.