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 ? 

...sorry >_<, I realised that this is not a bug but an issue for us.

This thing is that the <code>start_at</code> and <code>end_at</code> are both not null in db, so <code>schema_validations</code> help us validates the presence of these two values.

However, <code>before_validation</code> is called first and it found that those two fields are empty, so validation failed and <code>before_create</code> is not called.  Thus we cannot use <code>before_create</code> to set default values for not null fields, cause all of them are validated in both model and db level...