Version 0.3.1 Asyncio

Version 0.3.1 Asyncio:
support for async with context managers.
It is a breaking change.
You must close the underlying aiohttp client connections in either of two ways, via context manager:

async with btc:
    print(await btc.balance())

Or, if you don't use context managers, close it manually:

print(await btc.balance())
await btc.close()