Standalone install.sh requests root privileges even when it doesn't need to

The standalone install.sh script always uses `sudo`, even when it doesn't need to.

This pull request modifies the standalone install.sh to check for write permission on
the `/usr/local` dir before requesting root privileges with sudo.

Allows users with user-writable `/usr/local` directories (e.g.
most users of Homebrew on Mac OS X) to install as themselves.