Check for @data.url being nil

This fixes NoMethodError raised by @data.url[0] when @data.url is nil
Refactor Feed2Email::Config and rewrite spec

It is no longer a singleton and it has no knowledge of constants. Config
loading is performed with a class method.
Refactor Feed2Email::Logger and rewrite spec

It is no longer a singleton.
Add fuubar gem as development dependency
Mark required SMTP options in README file