Rp2

* added new pair conversion plugin infrastructure
* added support for currency conversion to transaction resolver (useful to support non-USD fiats from foreign exchanges)
* fixed PR 44: added configurable historical data behavior and improved caching
* small improvements to code and documentation

* fixed PR 35: added exchange_withdrawal transaction type to Coinbase plugin
* various documentation improvements
* updated RP2 dependency to 1.0.0

* fixed issue 34: sometimes Coinbase returns bad fiat data for crypto conversions (fiat amount of sale < fiat amount of buy). DaLI used to raise an exception: now it issues a warning and sets the fiat fee to 0
* Coinbase plugin: _process_transfer() refactoring

* fixed PRs 31 and 36: various improvements to the logic that reads prices from Web

* Coinbase Pro: fixed a bug which caused the total fiat value (including fee) to be incorrect for certain crypto conversions
* added developer FAQ

* fixed a regression introduced recently: in certain cases, some fees related to conversions would no longer be generated (thus causing small deductions to be ignored)
* added support for USDC: previously it was conflated into fiat and didn't generate a tab, now it does
* Coinbase plugin: added support for fiat_withdrawal and fiat_deposit transactions. They are now tracked internally and no longer cause an unsupported transaction warning
* Coinbase Pro plugin: added support for "conversion" native transactions (these seem to occur when CBPro converts from/to a stable coin)
* minor changes to documentation