* [GCE] Caching support for inventory script.
The GCE inventory script now supports reading from a cache rather than making the request each time. The format of the list and host output have not changed.
On script execution, the cache is checked to see if it older than 'cache_max_age', and if so, it is rebuilt (it can also be explicity rebuilt).
To support this functionality, the following have been added.
* Config file (gce.ini) changes: A new 'cache' section has been added to the config file, with 'cache_path' and 'cache_max_age' options to allow for configuration. There are intelligent defaults in place if that section and options are not found in the configuration file.
* Command line argument: A new --refresh-cache argument has been added to force the cache to be rebuild.
* A CloudInventoryCache class, contained in the same file has been added. As a seperate class, it allowed for testing (unit tests not included in this PR) and hopefully could be re-used in the future (it contains borrowed code from other inventory scripts)
* load_inventory_from_cache and do_api_calls_and_update_cache methods (, which were largely lifted from other inventory scripts, in a hope to promote consistency in the future) to determine if the cache is fresh and rebuild if necessary.
* A 'main' check, to support the script being imported and testable.
A new dictionary has been added to the list output, located at ['_meta']['stats'] that informs if the cache was used and how long it took to load the inventory (in 'cache_used' and 'inventory_load_time', respectively).
* fixed default value error; change cache time to 300
renamed plugins to contrib (they are not really plugins)
rewrote README.md to reflect new usage
added new dir to setup.py so it gets copied with installation, in views
of making using inventory scripts easier in teh future
