Multiple OR'ed conditions support

Multiple OR'ed conditions
-------------------------

You can add simple ORed conditions to the same WHERE clause using
``where_any_is``. You should specify multiple conditions using an
array of items. Each item will be an associative array that contains
a multiple conditions.

```php

    <?php
    $people = ORM::for_table('person')
                ->where_any_is(array(
                    array('name' => 'Joe', 'age' => 10),
                    array('name' => 'Fred', 'age' => 20)))
                ->find_many();

    // Creates SQL:
    SELECT * FROM `widget` WHERE (( `name` = 'Joe' AND `age` = '10' ) OR ( `name` = 'Fred' AND `age` = '20' ));
```

By default, it uses the equal operator for every column, but it can be
overriden for any column using a second parameter:

```php

    <?php
    $people = ORM::for_table('person')
                ->where_any_is(array(
                    array('name' => 'Joe', 'age' => 10),
                    array('name' => 'Fred', 'age' => 20)), array('age' => '>'))
                ->find_many();

    // Creates SQL:
    SELECT * FROM `widget` WHERE (( `name` = 'Joe' AND `age` > '10' ) OR ( `name` = 'Fred' AND `age` > '20' ));
```

If you want to set the default operator for all the columns, just pass it as
the second parameter:

```php

    <?php
    $people = ORM::for_table('person')
                ->where_any_is(array(
                    array('score' => '5', 'age' => 10),
                    array('score' => '15', 'age' => 20)), '>')
                ->find_many();

    // Creates SQL:
    SELECT * FROM `widget` WHERE (( `score` > '5' AND `age` > '10' ) OR ( `score` > '15' AND `age` > '20' ));
```
Merge pull request #201 from lrlopez/where_any_is

Multiple OR'ed conditions support