Fork, sixify & re-vendor pexpect-u about invoke HOT 5 CLOSED

Seems the fork-of-a-fork on Github has its own changes I don't want to bother with (esp given pexpect-u itself works fine for me right now) so I'm going to just import from hg into git, slap it up on my own Github for shits n giggles, sixify and vendorize.

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Ran this pexpect fork's test suite and about 5-6 seemingly minor things fail, things which don't seem like they're relative to my setup (e.g. the output of 'cat' and such). I.e. I'm wondering if they fail for the guy who forked it to begin with.

When I use this version in Invoke's test suite everything checks out at our level, so I think it's "good enough". Forging ahead.

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Realizing that the reorg done by the earlier fork probably doesn't impact us, pexpect/__init__.py doesn't import anything else but stdlib. So I'll probably just be able to vendorize that one file as I was doing before. Ho hum.

Have a bunch of stuff already cleaned up, main issue is (as usual) bytes vs unicode, since pexpect (or the fork) is trying to handle some of that already which actually just complicates things.

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Spoke too soon? Only had to remove all unicode checks/casts with six.string_type (after making sure that was the appropriate approach in each spot) and the Invoke suite is passing again.

Vendored the one file as above, seems to work OK.

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Travis passes too, we're done

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