Rp2

* minor documentation fixes

* fixed issue 20: Coinbase plugin now supports the staking_reward transaction type
* fix for Coinbase reporting fiat with low precision (only 2 decimal digits): if the value is less than 1c Coinbase rounds it to zero, which caused various computation problems (spot_price, etc.). As a workaround, when this condition is detected the plugin sets affected fields to UNKNOWN or None (depending on their nature), so that they can be filled later by the transaction resolver and RP2

* added threading support to Coinbase Pro
* thread count can now be specified in the plugin configuration section of the .ini file
* additional fix in missing fiat_fee issue in crypto swaps on Coinbase (issue 15)
* updated documentation

* fix for missing fiat_fee issue in crypto swaps on Coinbase (issue 15)

* -s CLI option to read missing spot prices from Web: rewrote implementation from Yahoo Finance (daily granularity) to Coinbase Pro (minute granularity)
* moved cache logic to its own module, so it can be used in multiple places: plugins, transaction_resolver, etc.
* fixed issue 19: the user needs to pass -s to read highly granular price data from Coinbase Pro, when it's missing (or computed as 0)

* merged PR 18: Coinbase plugin wasn't handling crypto conversions correctly and generated negative amounts.