v0.0.3-rc.15

Add support for authorized withdraws / deposits (#84)

Addresses #72, though this isn't quite tested yet.

- Provider classes now track info locally. So you don't have to pass `supportedAssets` to `fetchFinalFee`, but you do have to call `fetchSupportedAssets` ASAP.
- Switch back to all camelCase function inputs, since the Transfer stuff already outputs stuff in camelCase. Unfortunately, it means that devs might have to convert names from `info.fields` from snake_case to camelCase (and I think we should stop short of rewriting those values). But I think, better to be consistent and be clear about that consistency than to be correct but confusing.
- Add a function to set bearer tokens.
- Set Authorization headers when deposit/withdrawing an asset that's auth-required.