Add support for 'override' action.
-current
Merge pull request #3 from btorresgil/override

Add support for 'override' action.
spaces
fix: pan.xapi.rst:276: (WARNING/2) Title underline too short.
fix setting element for --override
missing element argument for --override
Add --validate to panxapy.py which runs commit with a cmd argument of
<commit><validate></validate></commit> to validate the configuration.
This is a new feature in PAN-OS 6.0.
document --sync, --interval, --timeout for panxapi.py
add example: Print operational command variable using shell pipeline.
add 'serial' varname to .panrc.  allows you to have tags which reference
a panorama managed device via redirection.  suggested by Jonathan Kaplan.

for example, given:

  hostname%0001C100522=10.0.0.1
  serial%0001C100522=0001C100522
  api_username%0001C100522=admin
  api_password%0001C100522=admin

$ panxapi.py -t 0001C100522 -Xjo 'show system info'|grep serial
op: success
        "serial": "0001C100522",

performs the API request on panorama managed device 0001C100522.