Minor updates/additions/fixed for Diamond

src/collectors/example/example.py
  doc Update for simpler way for testing a module, especially those that require
  running multiple times to gather metrics

src/collectors/files/file.py
  New FilesCollector that will collect metrics from file(s) placed in a pickup directory
  options:
   'dir':     'The directory that the performance files are in'. Default /tmp/diamond
   'delete':  'Delete files after they are picked up', Default False
   'path':    'additional prefix added to metrics collected'.  Default '.' (don't add a prefix)

  File format is just lines of
  metricname.metricname.whatever:10.2
  another.value:12345

src/collectors/cpu/cpu.py
  New option 'normalize' (default false).  When set to true, 'total' cpu metrics are divided
  by the number of cores (so that total is always 0-100%)

src/collectors/diskusage/diskusage.py
  New option 'send_zero' (default false).  When set to true, metrics are still sent to grapite
  even if there was no IO during the previous period

src/collectors/diskspace/diskspace.py
  New metrics for _percentfree and inodes_percentfree
new collector functionality
Merge pull request #472 from visioncritical/master

new collector options, new file collector, and minor doc update