Usability improvents

[PhillipSz]

Tell the user that Cython/afl is required, and if it is not installed tell the user to do so.
Don't just give an error tell the user what to do.

[jwilk]

For the record, this what you currently get if you try to build python-afl without Cython installed:

Traceback (most recent call last):
  File "setup.py", line 33, in <module>
    import Cython.Build
ImportError: No module named Cython.Build

And this is what you currently get if you run py-afl-fuzz without AFL proper installed:

/usr/local/bin/py-afl-fuzz: 13: exec: afl-fuzz: not found

In both cases, it's obvious to me what's wrong and how to fix it. But I'm probably biased.

But I've just realized that these prerequisites are not documented anywhere. I'll update the documentation shortly.

[jwilk]

In ff514a6, I documented build-time and runtime dependencies .
In ff1cdaa, I made the error message when afl-fuzz is missing more user-friendly.
In 53f23eb, I modified setup.py to catch ImportError.

Are you satisfied with these changes?

[PhillipSz]

yeah, thanks a lot.

[jwilk]

I've released python-afl 0.5.4, which includes the aforementioned changes.