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 ? 

actually, all main records should have timestamps and userstamps. I'm not so sure about soft deletion yet because soft deletion prevents us from using unique indexes properly. We should probably use something like paper_trail.