Adds getMonthsBetween

This new method allows to compute the number of months between two dates. Example usage:
```javascript
var sDate = new Date(  "Feb 28 2013" );
assert.equal( sDate.getMonthsBetween(  new Date( "Mar 30 2013" ) ).toFixed( 5 ), 1.09677 );
```
Note that the return value is always a decimal value even if two dates are identical. So the following code will return <code>0.03571</code> rather than <code>0</code>
```javascript
var sDate = new Date(  "Feb 28 2013" );
assert.equal( sDate.getMonthsBetween( new Date( sDate.getTime() ) ).toFixed( 5 ), 0.03571 );
```