[add] Page specific meta descriptions, the easy way

Rather than have duplicate page descriptions across all pages of the site it's beneficial to have page-specific meta descriptions. 

## Background
Matt Cutts says duplicate meta descriptions are not cool:

http://youtu.be/W4gr88oHb-k

## Proposed solution
We could set a unique description in each post front-matter and use that, but for the sake of laziness and ease-of-use it's far easier to grab the excerpt and use that instead. If there isn't one, fall back to the site description. 

### Testing
Check that the post 0000-00-00-welcome-to-jekyll now has it's own meta description, whilst the homepage uses  the site.description
Good catch @XhmikosR Thanks.