US-0001 - adding_proposal.feature complete implementation and test

Acá va la implementacion de la US-001
Agregadas las carpetas step_definitions y spec, dos clases del modelo con sus respectivos specs y los steps del feature adding_proposal.
Use 2 (not 1) spaces for indentation.<br>Do not use parentheses for method calls with no arguments.<br>Space missing inside {.<br>Space missing inside }.<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.<br>Tab detected.