set up talent's sign up path and controller

add linkedin gem
Set up LinkedIn model
Make TalentSignupHandler and User model with appropriate migration
Create LinkedinOauthSetting model and migration
Make model and migration for TalentProfile
Create model and migration for Skills
Create model and migrations for proficiencies
Merge pull request #5 from thisismetis/ag-af-linkedin-user-signup

User auth for Talents using LinkedIn API