course pages

depends on #20 and #24 

Changes in this PR:
1. install gem simple form
2. course pages and some default actions (new, create, edit, update)

This PR is not ready for merge, there are things need to be discussed:

The simple_form gem is not configured, are we confirm going to use bootstrap ? if that I will do <code> rails generate simple_form:install --bootstrap </code>

I think there are still some bugs in the schema_validations gem,  the <code>before_create</code> callback is not called in model unless I commented out the gem.. how would we deal with this ? 

if it's two different models, then save in a transaction. What's better is to use relations and let Rails automatically make the transaction (in that case you won't need an explicit transaction)