Use remotely hosted ZeroClipboard.swf

Don't include it in the repository due to licensing issues.
Remove ZeroClipboard.swf from docs

Substitute with CDN-hosted version. Tested on a https-hosted location to
ensure correct behaviour on an environment like our live website.
Minor changes to docs build

Add #ruby line to Gemfile so that current versions of rvm detect a
known-good Ruby version and a gemset. Modify _config.yml to exclude some
source code files from the built site.
Merge and close #371
Merge and close #376