Let trello filter out needless data

[ardumont]

The trello API permits to filter data.
This is API specific (board, card, checklist, item, action, etc...)

Exploit it to diminish the quantity of data to parse.

[ardumont]

Some work has already been done regarding this.

This is not complete but this is worth mentioning.

Since #100, an effort has been made to have the data received from trello transformed into "org-trello", i.e. the same structure as the data parsed by org-trello from the buffer.

This means that the data is parsed and transformed if it is needed.
Otherwise, it is dropped.

This means, that further manipulations benefit from it.
This also means that there is still network load since it is not trello that does the filtering.

So there remains work to be done. Indeed, executing a specific request on trello's side to only have the data needed will reduce greatly the data to transit through network and also reduce the parsing time.

[ardumont]

Also, #133 filtered out the data we did not need when retrieving the cards from the board (this is done when executing the 'sync from trello' command).