LibratoHandler: allow regex filtering of submitted measurements

This feature is particularly useful to limit usage to *only* desired measurements to keep bandwidth and metering costs down.

NOTE: I tried to mimick the exclude_filters behavior of the diskspace collector -- using the include_filters variable name; although I am joining the regexes from the list into a single string in order to get it to properly. Using the ```exclude_reg = re.compile(self.config['exclude_filters'])``` code from the diskspace module failed to handle a list. Perhaps this also needs to be checked out?  Please let me know if I can assist further.

Thanks,

~ Brice