Markusconfigurator

This is a pull request for simplifying MarkUs configuration. Now, specific configuration will be done in config/config.yml and not anymore in config/environments/<env>.rb. This is Rails best default way to do this.

I updated MarkusConfigurator in order to take into account config.yml.
Moreover, MarkusConfigurator doesn't set default values anymore. It was confusing in production. MarkusConfigurator will raise an error if a setting is missing.

This is the first part of the patch issued from Alan's (UofT) discussion. A second patch for routes with different instances on same subdomains will follow.

poke @danielstjules @karenreid 
Prefer {...} over do...end for single-line blocks.<br>Prefer `map` over `collect`.<br>Line is too long. [124/80]<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.