Make eliot optional

[exarkun]

This makes machinist also work when eliot is not installed.

Fixes #6.

[wallrj]

Thanks @exarkun

I think this looks good. Sorry this took a long time. I was confused by the duplicate HL and HC repositories and then further confused because I checked out your other eliot logging removal branch optional-eliot-6.

Notes:

• Ran tests locally - they pass
• Verified the correct skip messages were printed with different eliot installations.
• Checked for similar coverage between master and branch (although it was difficult because so much code has moved)
• Installed machinist using pip and the extras_require syntax - it worked.
• Visually checked that moved code had not changed.

I'd like to have run the docs/turnstile.py with and without eliot installed and seen the log output (or lack of), but that example seems to be missing a library.

(machinist)(master ? =)[~/.../HybridLogic/machinist]$ python doc/turnstile.py
Traceback (most recent call last):
  File "doc/turnstile.py", line 9, in <module>
    from turnstilelib import TurnstileController
ImportError: No module named turnstilelib

Please answer or address the inline comments and then merge, unless there are any other checks that you'd like me to do.

[exarkun]

I was confused by the duplicate HL and HC repositories

Sorry about that. Since I've only gotten negative feedback on this choice so far maybe I'll give up the idea.

I'd like to have run the docs/turnstile.py with and without eliot installed and seen the log output (or lack of), but that example seems to be missing a library.

... yea. That library doesn't exist, I made it up for the example. I'm going to write some real, runnable examples next so hopefully this issue will go away.

Thanks!

[coveralls]

Coverage increased (+0.87%) when pulling 6f60190acace5f8dc9eb22648d04c352b5057968 on hybridlogic:retry-optional-eliot-6 into 572d90c3d019cb428a1e2807a8e3419bcd37c953 on hybridcluster:master.