Async support

[faroit]

Support for async requests (e.g. via aiohttp) could make a nice improvement. Also it could speed up offset searches significantly.

Since pygbif has no support for legacy python, this could be implemented without much side effects.

[sckott]

Is your proposal to have async across all public functions or just for certain functions?

[faroit]

we could start with the most useful one which is probably occurances and see how it works out and how much better it performs.

[sckott]

sounds good