Update install documentation
Switch to kramdown to get section anchors
Swap prerequisites and get the code sections

The user probably needs to install git before they can get the code, so
swap these two sections over. Also some other minor formatting.
Updated copy for /about page with anchors
Switch to pretty permalinks
Update docs to work properly with kramdown
Add some more copy to the documentation home page
Expand the hosted popit documentation
Per-page meta description with global fallback

Each page can specify a description to go in the meta tag in its yaml
front matter, if there isn't one then the global one from _config.yml is
used.
Split out Using the API guide from the details API reference
Corrections to the MongoDB information in install.md
Remove old install documentation

It's now been condensed down to a single install.md document which goes
through all these steps rather than seperate them out into different
documents.
Need to manually install elasticsearch if using macports
Add section about running tests to install docs
Update vagrant installation instructions
Links between manual and vagrant install instructions
Add adding data page
Rename hosted to getting started and change sidebar
Replace self-hosted page with install instructions
Refer to popit directly rather than the documentation