Python 3 support, Progress & Speed Bar and replace `mechanize` with `requests`

I found when using `mechanize` to connect, the program always ends up with frozen for no reason. Also `mechanize`  currently doesn't support Python 3. Then I noticed pull #66 which suggests using `requests`. So I basically reproduce    @luk51000's work. Here are the list of my enhancements:

* Python 3 support with the help of package `six`.
* Progress bar and speed bar.
* Using `requests`, which seems has better QoS(Further tests may be required).

BTW, I'm sorry that I used PEP Auto-format Plugin to format the code on post-save.