Composite table keys with non-existent XPATH target now Null

When specifying a composite table key it has been observed that some of the values of the key may not exist.  This is seen in chassis hardware.
In the event of an unfound XPATH the key value in the tuple is now None
Original behaviour was an uncaught exception.
Added support for version_info into JSON and YAML
Fixed JSON support for RPC
version_info unit tests
unittest2 dev req
Merge pull request #298 from shermdog/table-missing-xpath

Composite table keys with non-existent XPATH target now Null
Added xml_rpc function to fetch XML RPC of a CLI command
Merge pull request #305 from shermdog/288

Fix for #288
Merge remote-tracking branch 'upstream/master' into cli-warning
Renamed to display_xml_rpc
Merge pull request #306 from shermdog/cli-warning

Added display_xml_rpc function to fetch XML RPC of a CLI command
added timeoutDecorator for execute (print still enabled)
Removed print debug from timeoutDecorator