WebUI: make API locale independet

Sizes are now given in bytes.
Dates are Unix timestamps and converted to ISO 8601 in the web UI.
Numbers are not converted to strings.
-1 is returned for undefined values.

Some keys have been splitted:

Torrent list (json/torrents)
 * num_seeds: number of peers seeding
 * num_complete: total number of peers seeding
 * num_leechs: number of leechers
 * num_incomplete: total number of leechers

Torrent generic properties (propertiesGeneral/hash)
 * total_uploaded: total amount of data uploaded
 * total_uploaded_session: data uploaded this session
 * total_downloaded: total amount of data dowloaded
 * total_downloaded_session: data downloaded this session
 * time_elapsed: number of seconds this has been active
 * seeding_time: number of seconds this has been active while complete
 * nb_connections: number of connections
 * nb_connections_limit: max number of connections allowed

Global transfer info (json/transferInfo)
 * dl_info_speed: current global downalod speed
 * dl_info_data: data downloaded during current session
 * up_info_speed: current global upload speed
 * up_info_data: data uploaded during current session

Closes #1524.
Remove unneeded parameters from helper functions for WebUI