Enable Parallel Hashing

[clbg]

Dear John:
Your treehash library is fantastic, I really loved it!
Your code is clean and powerful, and It's convenient to use. because I can install from pip no matter where i am.
And I noticed that , this treehash lib use only one thread to compute hash and read file in the mean time, so I thought maybe I can help to speed it up a little bit.
So I added a multithread trick to speed up hashing
using the main thread to read file, and hand out hashing jobs to thread pool.

Pros: faster speed
I tested a 37GB file using 4 hashing threads on my computer, the consumed time reduced from 4'23 to 1'39. That's really a speedup. (271%)

Cons:compatibility
I imported concurrent.futures to use the thread pool, which is only for python3 , so this PR is not compatible with python2

[jdswinbank]

Thank you for much for this useful contribution! I am travelling at the moment, so haven't been able to properly look at your work yet — I will do so when I return. I very much appreciate the time you took to submit a PR!