fix exception in log_in, by accepting json dates as timestamps
fix exception in log_in, by accepting json dates as timestamps

when requesting a bearer token, mastodon (more specifically doorkeeper)
returns an object with a created_at attribute which is a plain timestamp
unlike in most of mastodon's api:

{
    "access_token": "hunter2",
    "token_type": "bearer",
    "scope": "read write",
    "created_at": 1504824250,
}

halcy/Mastodon.py