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.