Add page title metadata

[ioalexei]

Fixes #159

• title metadata added to each page
• Header templates in _includes updated to replace generic <title> tags with {{page.title}} to pull from metadata
• Tested locally with Jekyll

[nickbearman]

Thanks @ioalexei
There is a lot going on in this PR. It's great to know you have tested locally in Jekyll.
I'm going to accept it, and keep my fingers crossed! If it all goes horribly wrong, I will probably have to just revert it for the moment, as I don't have a lot of time to troubleshoot.
Fingers crossed....!

[nickbearman]

Thanks @ioalexei
I think it all works. If there are any problems (anyone) please do shout!