Adds an endpoint for the login to hit

* keeps a dynamic segment to pass in the provider name
* the intention is to store sessions with access tokens
* login params from linkedin take this form

```
{
    "code"=>”temporary-access-code”,
    "state"=>”string used for securing purposes”,
    "controller"=>"sessions",
    "action"=>"create",
    "provider"=>"linkedin"
}
```
config db

This is not great practice, but acceptable because heroku will take
care of all the db permissions
adds a session model with db table