Allow a points only intersection on coordinates with bounds

Intersections treat coords with and without bounds differently. This sometimes trips up users if they don't know whether their data has bounds (especially with netCDF).

This change allow you to always perform a points only intersection whether or not it has bounds by setting points_only to True. If it has bounds, these are retained.
See below for use examples:

```
>>> cube=iris.load(file)
>>>
>>> new_cube = cube.intersection(longitude=(30, 50))
>>> print new_cube.coord('longitude')
DimCoord(array([ 27.5,  32.5,  37.5,  42.5,  47.5,  52.5]), bounds=array([[ 25.,  30.],
       [ 30.,  35.],
       [ 35.,  40.],
       [ 40.,  45.],
       [ 45.,  50.],
       [ 50.,  55.]]), standard_name='longitude', units=Unit('degrees'), coord_system=GeogCS(6371229.0))

>>> new_cube = cube.intersection(longitude=(30, 50, True, True, True))
>>> print new_cube.coord('longitude')
DimCoord(array([ 32.5,  37.5,  42.5,  47.5]), bounds=array([[ 30.,  35.],
       [ 35.,  40.],
       [ 40.,  45.],
       [ 45.,  50.]]), standard_name='longitude', units=Unit('degrees'), coord_system=GeogCS(6371229.0))
```

Problems:
- if doing points only intersection on cube with bounds, it does (points = wrap_lons(coord.points, minimum, modulus)) twice. First time to get the indices of the cells we want in the intersection, second time in case there were split cells.
- is there a better way to call it than using "points_only"

No tests are included as I wanted feedback on whether I should make any changes
Does that not allow ignore_bounds to be set to anything (other than False or None) and it will still ignore the bounds?

I made that change then tried the following:

```python
>>> new = cube.intersection(longitude=(20,30), ignore_bounds=True)
>>> new
<iris 'Cube' of air_temperature / (K) (time: 147; latitude: 36; longitude: 2)>
>>> new = cube.intersection(longitude=(20,30), ignore_bounds=False)
>>> new
<iris 'Cube' of air_temperature / (K) (time: 147; latitude: 36; longitude: 4)>
>>> new = cube.intersection(longitude=(20,30), ignore_bounds='to')
>>> new
<iris 'Cube' of air_temperature / (K) (time: 147; latitude: 36; longitude: 2)>
```

When ignore_bounds was set to a string, it gave the same result as if it were set to True.