install.sh: install triangle from package sources
install.sh: clone PyOP2 and run tests unprivileged
README: quick install is not for PyOP2 developers

Also note a pitfall when piping the installscript into `sudo bash`.
install.sh: Add missing dependencies decorator, PyYAML
install.sh: Add some documentation
Merge pull request #258 from OP2/install-script

Tweaks to the install script and README referring to it

* Clone PyOP2 and run tests unprivileged, set $HOME to the user's
* Adds missing dependencies on decorator and PyYAML
* Clarify that quick install is not intended to be used by developers

Closes #257

Unfortunately notes and warnings are not properly rendered on GitHub
due to a bug: https://github.com/github/markup/issues/68