Added basic process and system metrics

These can be run via a python shell or a simple python script, but
the typical run_monitor interface is not expected to work on windows
systems due to a handful of differences between the platforms.
Sample smoke test

This is expected to move and/or be developed further.  It is used to avoid
the various issues that using the run_monitor interface on a windows system
is currently presenting.

From a windows command prompt:

'''
 $ cd \path\to\scalyr-agent-2
 $ python test_windows_monitor.py
'''
Removed unavailable CPU metrics
Added network and disk io counters
Dropped metric definition template
Preparing to change metric handler to an iterable.
Added disk usage

This required changing the gather sample functionality slightly; the
dispatch functions are expected to return an iterable object.
Quick clean up and test before commit