Provide "GitHub-like" anchor links affordance

Live preview: http://bitcoindevtest2.us.to/en/developer-guide

Yet [another good idea from @cbeam](https://github.com/saivann/bitcoin.org/issues/62). This pull request adds anchor link icons for all titles on devel-guide and devel-ref, which appear appear on mouse :hover similarly to READMEs or Wikis hosted on GitHub. This is achieved with some javascript code that automatically inserts the appropriate HTML inside all titles with an id. 

I've reused a previous icon. Just let me know if you think we should really use something similar to GitHub (chain icons seemed a little bit hard to make with inkscape so I gave up at the first try).