Use array.array instead of struct.iter_unpack
Use array.array instead of struct.iter_unpack

By using array.array instead of struct.iter_unpack I was able to get
over a 2x speedup in the `make_digest` function. I'm not sure if the
lack of array.array having support for endian-ness (e.g. the `<` in `<H`
previously used in `struct.iter_unpack`) will be a problem, but it seems
to be working on both the MBA and my RPi.

FWIW, my test script and results:

```
import timeit

setup = """from eyeflask.server.crypto import make_digest
infile = ("/Users/n8henrie/Desktop/eyeflask_uploads/integritydigest/"
          "IMG_0207.JPG.tar")
f = open(infile, "rb")
"""

print(min(timeit.repeat(stmt='make_digest(f, "6a147b8986f6f2227361ac160a17e13d")',
                    setup=setup, repeat=5, number=1000)))
```

MBA
2.2 GHz Intel Core i7
8 GB 1600 MHz DDR3

number=1000, repeat=5

min with array.array:
	0.00620448193512857
min with struct.iter_unpack:
	0.014997487072832882

Raspberry Pi B+
700 MHz
512 MB RAM

min with array.array:
	0.48377920303028077
min with struct.iter_unpack:
	1.1263071660650894

MBA struct.iter_unpack / array.array
=0.014997487072832882/0.00620448193512857 == 2.417202

Pi struct.iter_unpack / array.array
=1.1263071660650894/0.48377920303028077 == 2.328143

Pi / MBA array.array:
=0.48377920303028077/0.00620448193512857 == 77.97253

Pi / MBA struct.iter_unpack()
==1.1263071660650894/0.014997487072832882 == 75.09972

n8henrie/eyeflask