Bump version to 0.5.7

aio: Only close future if it's not done or cancelled

Bump version to 0.5.5

Fix cert and error buffer when calling curl_easy_reset

Reset curl after performing, fix lexiforest#39

Clear headers after async perform

Bump version to 0.5.1

Add support for asyncio (lexiforest#33)

- Preliminary support for asyncio
- Removed curl header files from source, use make to build wheels

Refactor write_callback, change internal buffer to a stream-like call…

…back, fix lexiforest#25

Previously, instead of setting a python buffer to receive data by
curl's builtin write callback, we use a fixed callback function
and buffer to receive data, and after the full response content
is received, the data is copied back to user's buffer.

In the previous way, memory is wasted. More importantly, it's just
not possible to implement streaming.

Bump version to 0.3.8