Provider fallbacks

fixes to incorrect Provider addition, added duplicate checking when adding providers, added workarounds for the default provider in tests

Custom providers

Raw mode and cached chaining

added options to set amount of currency after initialization, and to instantiate Convert without a starting amount(raw/cached mode)

added functionality for semi-chains, that allow for retrieval of a raw rates object without being confined to a pair of currencies

Coverage

added result and error handling tests for APIs behind paywalls (those which do not get tested along with the others)