Test and document use of GraphQL-core with uvloop

In #141 it was discussed whether using a different event loop such as `uvloop` can speed up async use of GraphQL-core.

We should do some benchmarking and document how to replace the default event loop with a more efficient one. However, it should still be possible to use GraphQL-core as a pure-Python function, i.e. without depending on libraries that need to be compiled, such as `uvloop`.