Python 3 support, unpause command, & minor fixes

[juntalis]

Not sure how you feel about maintaining Python 3 compatibility, but I figured I'd offer the changes and see if you want any of them. Let me know if you'd like me to split something out. Brief overview of the changes:

• Noticed there was no corresponding unpause command so I added it in.
• --debug wasn't being used for anything. Saw the DEBUG field on the Vmrun class, so I linked them up.
• Added [options] to the scp command for consistency with the ssh command.
• Added a RuntimeError to Vmrun when it's unable to locate the vmrun executable.
• Changed the vmx file write mode to binary in rewrite_vmx to resolve a Windows-specific issue: Not sure on the Python 2 behavior, but in Python 3, text mode resulted in the os.linesep being replaced with a \r\r\n.
• Python 3 compatibility stuff adds a dependency on six. (Seemed cleaner than creating a mech.compat module)
• In the Mech.scp method, the "download" logic branch can potentially run even when src_is_host is False. (see example invocation below for details) I wasn't sure if this was behavior was intentional, so I suppressed the first character stripping when src_is_host == False.

mech scp ubuntu-vm /home/mech/file.txt file.txt

would've previously tried to download home/mech/file.txt.

Note: Tested each command in Python 2.7 and 3.6, but only on Windows. (didn't have a Linux dev environment handy when I was writing the changes)

[ColdHeat]

Hey there @juntalis! You should check out the devel branch https://github.com/mechboxes/mech/tree/devel.

A lot of rewrites and it should be compatible with Python 3. There are some unimplemented commands there too but it is a little more complicated to jump into than the master branch.

[juntalis]

Woops - completely overlooked that branch. Will check it out and see if any of my changes still apply. In any case, closing this pull request.

[Kronuz]

Yes, please check devel branch. It’s not Python 3 compatible yet, we need to make it compatible though. We can either use six or add a compat module. Please make a new pull request for that branch if you can.