Allow configuration location to be configurable

This change allows both the `deploy_config_path` and `stage_config_path`
to be moved from the default locations of `config/deploy.rb` and
`config/deploy` respectively.  These values __must__ be set in the
`Capfile` prior to `capistrano/setup` being called, for example:

    set :deploy_config_path, 'app/config/deploy.rb'
    set :stage_config_path, 'app/config/deploy'

    # Load DSL and Setup Up Stages
    require 'capistrano/setup'

Fixes #610
Merge pull request #684 from capistrano/feature/configurable-config

Allow configuration location to be configurable