First pass at a simple metric for variable objects

This is a very simple metric that simulates sinusoidal
varibility and tries to recover the period of the
variability using the peak value of a Lomb-Scargle
periodogram.  The data are sampled using the cadence
of the input OpSim run.

Since this is just a demonstration and is intentionally simple, there
are several limitations:
1. The periods explored in the periodogram
are windowed to be +-1 day of the possible periods.
2. No attempt is made to simulate the errors on the measurements.
3. This reports the guess at the period not the error in the period
relative to the input period.
Added docstrings for the variability metric.
Add import to __init__.py

This will bring any metrics added to varMetrics.py into
the mafContrib namespace.
Added a simple driver script

To run do:
$> cd examples
$> runDriver.py varMetricExample.py
Response to Lynne's suggestions

Corrected typo and changed numpy import to
the standard naming convention.
Merge pull request #6 from LSST-nonproject/issue/3

Issue/3