OPAM for dependency management

* Update docs
* Add list of requirements
Use gh-pages branch of pplacer-opam-repository

Negates git requirement; downloads packages directly from
GitHub, which should prevent broken hyperlinks.
Remove old install script
Merge remote-tracking branch 'origin/293-opam'

Closes #293
Fix link in README
Merge branch 'master' into dev

Conflicts:
	CHANGELOG
	docs/_static/install_pplacer.sh
Fix some links