client: add a timeout parameter on most Api methods

[greenknot]

Api.get_next_event() and Api.wait_for_text_event() can be stuck forever in case of unexpected behaviors from the device. There should be atimeout parameter to prevent that. I tried hard to implement a timeout feature on requests.Response.raw.readline() with:

• threading.Timer which close the socket after the specified amount of time has passed, but it doesn't make requests.Response.raw.readline() fail
• select.poll on the request socket and it doesn't work either

(As a side node, I think HTTP requests should also timeout after some time, but it can be done easily with requests.)