CRM_Utils_SQL_Select - Allow fluent query execution

CRM_Utils_SQL_Select - Allow fluent query execution

When I first wrote `CRM_Utils_SQL_Select`, I was a bit dogmatic about
loose-coupling and wanted the class to be entirely independent of the SQL
runtime. But this is a bit annoying in usage and training.

Before
======

To build and execute query, you had to pass the rendered SQL to the execute
function, eg

```php
$select = CRM_Utils_SQL_Select::from('mytable')
  ->select('...')
$dao = CRM_Core_DAO::executeQuery($select->toSQL());
while ($dao->fetch()) { ... }
```

After
=====

You can use more fluent style:

```php
$dao = CRM_Utils_SQL_Select::from('mytable')
  ->select('...')
  ->execute();
while ($dao->fetch()) { ... }
```

And you can chain with other DAO functions like `fetchAll()` or
`fetchValue()`.

```php
$records = CRM_Utils_SQL_Select::from('mytable')
  ->select('...')
  ->execute()
  ->fetchAll();
```