Init async uploader with queue

Queue based async uploader

References #98

Test case is:

c = PodClient()

data = b'abc123' * 5000000
data2 = b'abd123' * 5000000

c.create(File(sha256=sha256(data).hexdigest())) # data file item

for i in range(100):
    if i % 2 == 0:
        c.upload_file_async(data, cb)
    else:
        c.upload_file_async(data2, cb)