Refactor and Cleanup Code in aspen.rb File

Recommending a few code changes via this pull request:

- DRY out code by using an array to store DIRECTORIES.
- Update regular expression to match on first character (for now).
- Removed 'require_relative' writing (as no models had been created).

Recommending a few clerical changes via this pull request:

- Move hierarchy discussion to README.md file.
- Remove comments from file (as gem is published).
- Make final output a little clearer to read on command line.
- Rearrange code to follow execution flow of the program.

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This is what @chokim and I did during our lunch break today. :-)

_EXCELLENT_ work on publishing this gem.
Way to make us @flatiron-school alums proud!