Updated installation instructions

Differentiates between the stable package, develop branch, and other
possible versions. Includes add-on code for GUI installation and
additional git instructions for doing it that way. Mentions the build
dependency on pyuic4.
Install/symlink fix for systems with BSD mktemp

When using mktemp on a system with BSD userland utilities (like Mac OS X
has), a "template" must be provided (i.e. a vague desired filename for
the temporary file, e.g. /tmp/conf.db.XXXXXXXXXX). Linux works with or
without the "template", and is not harmed by its presence.
Install/symlink check for parent directory

When running the install or symlink scripts, care should be taken to
ensure that the target directory is not a parent directory of the
package itself.

This can happen accidentally if one already has a symlink from the
addons directory to the project directory, changes into the symlinked
project directory, forgets about being in the symlink, and then asks for
a symlink/install again back to the addon directory.
Correct handling for optimized Python bytecode

Adds support for ignoring "optimized" Python bytecode (.pyo) files and
also correctly clean them up during install and symlink requests. These
files are generated instead of the regular Python bytecode (.pyc) files
on Mac OS X (and maybe other environments, too).
Switch cache handling in preferences to listdir()

Previously, this was being handled by the Python glob module, but this
module does not seem to be available on the Mac OS X installation of
Anki.