remove hard dependency on fog

Load fog only if needed, and emit a helpful message if it's missing.

This removes a ton of dependencies, which makes the default installation
easier for newcomers.
add example for storing config on S3
mention config examples in the README
mention pointing the dash to the Riemann server ws port

This frequently confuses first time users.
fix unclosed html tag
document 'r' and 'p' keybindings
remove a dependency on an unused gem
use webrick by default, document thin/puma installation

Rationale: installing thin/puma needs a working toolchain and various
libraries installed. Failing that, "gem install" will abort with an
error which doesn't give much hints to people not familiar with ruby.
Webrick is much slower, but it's pure ruby and doesn't depend on any
other gem. Sinatra will pick up what is available, in this order of
preference: thin, puma, mongrel, webrick.
Merge pull request #67 from mfournier/small-details

Several small details