Adds support for async/await API calls (no deps)

[karthanistyr]

BREAKING:
Drops support for python 2.7 and < 3.5 (it relies on asyncio (so 2.7 is out), and python versions below 3.5 have different behaviours regarding the thread pool, see: https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.run_in_executor).

I am suggesting this change for your consideration.
Backwards compatibility has been preserved by running the synchronous kanboard.client.Kanboard.execute method into an asyncio executor (for now, only the default executor can be used, but the event loop can be customised via Kanboard() ctor)

Basic usage is to use the coroutines mapped by method calls with the _async suffix.
Example: for a given kb object (of type Kanboard)
Synchronous: kb.create_project(name="My project")
Asynchronous: kb.create_project_async(name="My project")

Fixes #14 (replaces PR #15)

[karthanistyr]

There are no added dependencies with this take on the async/await compatibility.
Some real world data:

sample_program.py

import kanboard
import time
import asyncio

kb = kanboard.client.Kanboard("[REDACTED SERVER URL]",
                              "jsonrpc",
                              "[REDACTED API TOKEN]")

# sync
t1 = time.time()
results = [res for res in [kb.get_all_users(),
                           kb.get_all_groups(),
                           kb.get_all_projects()]]
t2 = time.time()
print("Synchronous mode:", t2-t1)

# async
loop = asyncio.get_event_loop()
t3 = time.time()
results = loop.run_until_complete(asyncio.gather(
                            kb.get_all_users_async(),
                            kb.get_all_groups_async(),
                            kb.get_all_projects_async()))
t4 = time.time()
print("Asynchronous mode:", t4-t3)

$ python sample_program.py
Synchronous mode: 3.0287256240844727
Asynchronous mode: 1.368499755859375

[fguillot]

LGTM. Thanks!