JSON Output

This commits adds 2 things:
 - Returns a JSON object if the json parameter is defined in the URL (example: http://<ip>:<port>/?json,
   http://<ip>:<port>/processes?node=localhost&json)
    - 'page' and 'is_xhr' are not return in the JSON object (not needed)
    - I added the field 'timestamp' to the JSON object (it contains the timestamp of the response).
 - Add http://<ip>:<port>/process/<pid>/all, it returns a JSON object which contains all sections (Overview,
   Environment, Threads, Open files, Connections, Memory maps, Children, Limits)

I added the JSON output, because I plan to use psdash to monitor some hosts. Some python scripts will send HTTP requests to psdash. The data will be keept and plot (like mrtg).
In order to not make several HTTP Requests to get the process info, I added http://<ip>:<port>/process/<pid>/all