Allow multiples values for a single key in _buildWhere()

Allow user to set multiples values for a single key.

Examples : 
===
```
$posts = $this['db']->select('articles', '*', array('year' => array(2014, 2015)));
```

Tests : 
===
Empty string : `_buildWhere()` 
```
array(2) {
  [0]=>
  string(0) ""
  [1]=>
  array(0) {
  }
}
```
No parameters : `_buildWhere('year = 2014')` 
```
array(2) {
  [0]=>
  string(17) "WHERE year = 2014"
  [1]=>
  array(0) {
  }
}
```
Bind an integer : `_buildWhere(array('year' => 2014))` 
```
array(2) {
  [0]=>
  string(14) "WHERE year = ?"
  [1]=>
  array(1) {
    [0]=>
    int(2014)
  }
}
```
Bind a string : `_buildWhere(array('year' => '2014'))` 
```
array(2) {
  [0]=>
  string(14) "WHERE year = ?"
  [1]=>
  array(1) {
    [0]=>
    string(4) "2014"
  }
}
```
Bind an array : `_buildWhere(array('year' => array('2014', '2015')))` 
```
array(2) {
  [0]=>
  string(19) "WHERE year IN(?, ?)"
  [1]=>
  array(2) {
    [0]=>
    string(4) "2014"
    [1]=>
    string(4) "2015"
  }
}
```
Bind a string plus an array : `_buildWhere(array('year' => '2014', 'month' => array('jan', 'fev')))` 
```
array(2) {
  [0]=>
  string(33) "WHERE year = ? AND month IN(?, ?)"
  [1]=>
  array(3) {
    [0]=>
    string(4) "2014"
    [1]=>
    string(3) "jan"
    [2]=>
    string(3) "fev"
  }
}
```
Bind an array plus a string : `_buildWhere(array('year' => array('2014', '2015'), 'month' => 'jan'))` 
```
array(2) {
  [0]=>
  string(33) "WHERE year IN(?, ?) AND month = ?"
  [1]=>
  array(3) {
    [0]=>
    string(4) "2014"
    [1]=>
    string(4) "2015"
    [2]=>
    string(3) "jan"
  }
}
```
Bind two arrays : `_buildWhere(array('year' => array('2014', '2015'), 'month' => array('jan', 'fev')))` 
```
array(2) {
  [0]=>
  string(38) "WHERE year IN(?, ?) AND month IN(?, ?)"
  [1]=>
  array(4) {
    [0]=>
    string(4) "2014"
    [1]=>
    string(4) "2015"
    [2]=>
    string(3) "jan"
    [3]=>
    string(3) "fev"
  }
}
```