INSTALL.md: add instructions for using Python 3.

At least for *nix-like systems.
INSTALL.md: 2 to 3 or vice versa, remove executable Supybot* first. Thanks nyuszika7h :)
Merge pull request #553 from Mkaysi/install3

INSTALL.md: add instructions for using Python 3.
Add Utilities as a default plugin.
Fix precedence of commands by removing Utilities from the list of important plugins.
Add some Debian files to .gitignore

And remove the entry for Emacs, which doesn't work due to being a comment anyway.
Less aggressive ignoring of vim swap files

It's a rare sight to see non-.swp swap files anyway, and we don't want
other files (e.g. .sql) being accidentally ignored.
Add nano.save to gitignore
Add .pyd to gitignore [skip ci]

.pyd is a rare sight, but I added it just to be on the safe side.
Merge pull request #554 from nyuszika7h/debian-gitignore

Add some Debian files to .gitignore
ChannelStats: Remove useless space.

Closes #170.
[ci skip]
Merge pull request #558 from nyuszika7h/issue-170

ChannelStats: Remove useless space.
Unix: Fix FD leaks and Windows compatibility
Unix: Add @shell command
ircdb: Honor ignoreDefaultAllow for unknown users too.
Update README.md

Fix typo
Fix typo in the README.
Add support of IP-binding in non-IRC connections.
Add support for SRV records in domain name resolution.
Clarify help text for !shell

I should stop using @, poor @shell getting notified by commits
irrelevant to his interests. :P

[skip ci]