Adds support for filtering out non float values before sending

[askoog]

To avoid server side parse errors, do not bother sending string values or null to graphite.

Added xml configuration value "filterNonFloatValues" that defaults to false

[cyrille-leclerc]

@askoog sorry for the delay. Thanks for you submission. Unfortunately, the build fails on unit tests. Did you have a chance to look at this failure? https://travis-ci.org/jmxtrans/jmxtrans-agent/jobs/439558727