gpsdio info command

[geowurster]

We need a better info command. The rio/fio info model works well, and allows the user to get serialized stats as JSON or individual members.

Needs to report:

• [x] Spatial extent as xmin ymin xmax ymax
• [x] Is the file sorted?
• [x] MMSI histogram
• [x] Message type histogram
• [x] Total number of messages
• [x] Number of unique MMSI's
• [x] Number of unique message types
• [x] Min timestamp
• [x] Max timestamp

The MMSI histogram could easily be thousands of lines long if run on a large chunk of data so it should be off by default. Need to decide if the types histogram should be off by default.

[geowurster]

Currently under development in an info branch.