Cleanup config handling, redux.

Rebase of [PR #1147](https://github.com/imathis/octopress/pull/1147):

* Hoist a couple bits of config out of config.rb to keep configuration in one place.
* Make Sinatra use config instead of duplicating config setting via hard-coding.
* Make config hashes stringify/symbolify recursively (handy for namespacing).
* Make Octopress::Configuration use its own read_config method from read_configuration.
* Make read_config smarter about absolute vs. relative paths.
* Built test case around recursive stringification/symbolification of hash keys.

Also:

* Converted test cases to RSpec.