Added config/application.yml to .gitignore

This file shouldn't exist anymore since we're using .env but adding it to .gitignore to prevent any future leaks.
Implement Payable concern for Events
Whitespace: tabs -> spaces
Set up generic payable model
Implement Payable concern for Events
Set up generic payable model
Updated associations, made payable / payment association polymorphic, modified routes

Currently having trouble with form_for with a polymorphic association;
the form_for helper should take an array of [ payable, payment ] but currently
complains of an undefined local variable or method payments_path.
See app/views/payments/_form.html.erb
Fixed borked Gemfile.lock
Whoops, the bug was caused by payments_path in the old new.html.erb
Updated Rails to 4.1

Added user status to users: registered or active

* Users are registered by default
* Users are active once they have paid the year's dues
Added Spring gem.

Added Spring to development and test environments. Same with Rspec. Replaced mock with double to fix deprecation warnings. Updated travis Ruby version to 2.1.1.