create initial data structure for the 3Bot capacity order prototype

Story: https://github.com/threefoldtech/home/issues/52

Goal of this issue is to already provide a python class that can be used to a encode and decode a 3Bot capacity order, such that it can be passed as binary data together with a send_money tfchain method call.

The order can contain the following information:

resource units (wiki: https://threefoldfoundation.github.io/info_grid/#/concepts/resource_units):
- CRU (Core Units), expressed in units of cores, fixed uint8;
- MRU (Memory Unit), expressed in GB, fixed uint16;
- HRU and SRU (HDD and SSD), expressed in GB, fixed uint32;
- NRU (Network Unit), expressed in GB, fixed uint16 (probably too big);
duration: amount of (30-day) cycles to reserve for, fixed uint16 (giving a really big upper limit);
description: max 32 bytes, optional but possible;

threefoldtecharchive / jumpscale9_lib

create initial data structure for the 3Bot capacity order prototype #259