Add an API for exists().

If I want to check if something exists currently I would do something like this:

```php
CmsCompanyQuery::create()->count() > 1;
```

Executes

````sql
SELECT COUNT(*) FROM cms_company;
````

But `count()` is expensive on larger tables, so I'd need to keep in mind to limit it:

````php
CmsCompanyQuery::create()->limit(1)->count() > 1;
````
Which surprisingly produces
```sql
SELECT COUNT(*) FROM (SELECT cms_company.com_id, cms_company.media_id, cms_company.admin_id, cms_company.com_name, cms_company.com_email, cms_company.com_address, cms_company.com_address2, cms_company.com_postal, cms_company.com_country, cms_company.com_employee, cms_company.com_skin, cms_company.com_theme, cms_company.com_activity, cms_company.com_status, cms_company.com_created, cms_company.com_modified, cms_company.com_creator, cms_company.pay_id, cms_company.pay_amount, cms_company.com_last_payment, cms_company.com_next_payment, cms_company.is_reward, cms_company.vote_start, cms_company.vote_end, cms_company.is_banner, cms_company.is_goal, cms_company.skin_path FROM cms_company LIMIT 1) propelmatch4cnt
```
Limiting the select column
```php
CmsCompanyQuery::create()->select(['com_id'])->limit(1)->count();
```

Turns into:

```sql
SELECT COUNT(*) FROM (SELECT cms_company.com_id AS "com_id" FROM cms_company LIMIT 1) propelmatch4cnt
```

Which is still not ideal because it's quite convoluted and the SQL query is still getting table data unnecessarily. A faster way would be to add an `exists()`:

```php
CmsCompanyQuery::create()->exists();
```
Produce.
```sql
SELECT 1 FROM cms_company LIMIT 1;
```
Which should be the fastest query to check for existence since we aren't interested in retrieving any data.