Switch from using environment variables to yaml config.

- Load the yaml config in the Rakefile and config.ru for Sinatra app.
- Store redis connection options in the yaml config and load.
- Store the nikeplus data in the yaml config and load.
Add unicorn dependency to Gemfile.

- Will be using unicorn on the server to run the Sinatra app behind nGinx.
Use Constants in entire project.

- Move common strings from Rakefile to Constants class.
- Use Constants in rackup file, Rakefile, and Steps app.
Do not set timeframe high/low if 0.
Set the date formats to be consistent.
Allow for customizable number of graph days.

- Configure the number of days to go back from current date to get data.
Use Constants to set high/low stats key value pairs.