Implementing Mentoree, Adding ex_machina

[lbighetti]

• User has 1 Mentoree

This means a user is a mentoree, just as it is a mentor (and these 'roles' can be active or not)

• Added Mentoree to Mentorship context

• Not adding delete

Again we'll go with is_active true or false instead of deleting.

[lbighetti]

This resolves #17

[lbighetti]

I'll add ex_machina to do this.