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)

added converter.active test + reduced provider setup complexity by removing keyNeeded (if %KEY% is present in the template url string, the stored key will be inserted)

Error handling