Sql as 'option' type for JFormFieldList

Allows you to create `<option>` tags for `JFormFieldList` or any class that inherits from it which basically function exactly like `JFormFieldSQL`. This essentially makes `JFormFieldSQL` obsolete and adds its functionality to all classes that extend `JFormFieldList`. And, since it's built on top of #1115, that means you can create `checkbox` or `radiobutton` inputs based on SQL. 

An example of the `sql` field type from the wiki (http://docs.joomla.org/SQL_form_field_type) looks like this:
```xml
<field name="title" type="sql" default="10" label="Select an article" query="SELECT id AS value, title FROM #__content" />
```
But could now be rewritten as:
```xml
<field name="title" type="list" default="10" label="Select an article">
    <option type="sql" query="SELECT id AS value, title FROM #__content" />
</field>
```
Which might not seem better except you can also do this:
```xml
<field name="title" type="checkboxes" label="Select some articles">
    <option type="sql" query="SELECT id AS value, title FROM #__content" />
</field>
```

Also, I've added this:
```xml
<field name="title" type="checkboxes" label="Select some articles">
    <option type="sql" query="SELECT id AS value, title FROM #__content" >
        <query driver="mysqli" query="SELECT `id` AS `value`, `title` FROM `#__content`" />
        <query driver="sqlsrv" query="SELECT [id] AS [value], [title] FROM [#__content]" />
    </option>
</field>
```
Which makes it so that the `option`'s query will be used by default but, if there is a query for the specific database driver being used, that one will be used instead.