Add basic support for importing and exporting rules to/from JSON

[pmarkowsky]

This PR adds support for importing and exporting rules to/from a JSON file when Santa is not configured to use a sync server.

Fixes #56.

The format of the file is:

{"rules": [{ "policy" : "BLOCKLIST",
                   "rule_type" : "BINARY",
                  "identifier" : "84de9c61777ca36b13228e2446d53e966096e78db7a72c632b5c185b2ffe68a6"
                  "custom_url" : "",
                  "custom_msg" : "/bin/ls block for demo"}
               ]}

Example usage of export

$ sudo santactl rules --export rules.json

Example usage of import

$ sudo santactl rules --import rules.json