makes user_id field nullable

If a user attempts to sign in with an account that does not exists, the exception below is thrown, because Sentry attempts to update the throttle table with a null user_id. This makes the user_id field nullable.

```
Illuminate \ Database \ QueryException
SQLSTATE[HY000]: General error: 1364 Field 'user_id' doesn't have a default value (SQL: insert into `throttle` (`type`, `updated_at`, `created_at`) values (global, 2014-03-27 08:55:12, 2014-03-27 08:55:12))
```
Updated documentation links
Update Group.php
Let ServiceProvider set the configured user model
Make userModel configurable for the ServiceProvider
Merge pull request #403 from mikevrind/master

Let Eloquent Throttle use the configured user model
Merge pull request #395 from AnasT/patch-1

Update Group.php
Merge pull request #390 from Kartoshka/master

Updated documentation links
Merge pull request #386 from dberry37388/patch-4

makes user_id field nullable in throttle table
Minor docblock fix
Use freshTimestamp instead of DateTime
Merge pull request #414 from lucasmichot/docblocks

Minor docblock fix
When adding or removing groups on a user object, the merged permissions cache should be cleared.
update docs links
compare hashes using slowEquals

Signed-off-by: Suhayb El Wardany <me@suw.me>
make the BaseHasher abstract, final slowEquals

Thanks @shadowhand

Signed-off-by: Suhayb El Wardany <me@suw.me>
Merge pull request #415 from lucasmichot/laravel-freshtimestamp

[ELOQUENT] Use freshTimestamp instead of DateTime
modify getPassword so it respect getPasswordName

when trying to extend this model, and using custom password field, it not respect overrided getPasswordName() 

e.g. I want to change password field to other field
```php
class ProviderCredential extends Cartalyst\Sentry\Users\Eloquent\User {

    public function getPasswordName()
    {
        return 'OtherThanDefaultPasswordField';
    }
}
```
Merge pull request #444 from xsanisty/master

modify getPassword so it respect getPasswordName
Delete changelog.md