added --dropnulls and --dropzeroes options to whisper-fetch.py

These options helps to cut down on the whisper metrics data export file size on my particular data series, which contains a lot of useless nulls and zeroes.

[PEP289](http://legacy.python.org/dev/peps/pep-0289) sez: "List comprehensions greatly reduced the need for filter() and map(). Likewise, generator expressions are expected to minimize the need for itertools.ifilter() and itertools.imap()".

Using `itertools.ifilter` here totally works, but generator expressions have been in python since 2.4 so should be preferred.