Add  Skycoord.to_pixels_wcs

This is a fairly trivial add on to #2885 that adds a convinence method to go from SkyCoord to pixel coordinates.

Not that there is *not* an inverse method, because it would make most sense as a classmethod, and `astropy.wcs.utils.pixel_to_skycoord` only uses the `SkyCoord` object explicitly.  It wouldn't be too hard to change that, though... What do you think, @astrofrog?
Imports do add a non-trivial overhead even if it's for modules that have already been imported:

```
In [1]: import astropy

In [2]: def test1():
   ...:     return 1 + 1
   ...: 

In [3]: def test2():
   ...:     import astropy
   ...:     return 1 + 1
   ...: 

In [4]: %timeit test1()
10000000 loops, best of 3: 95.9 ns per loop

In [5]: %timeit test2()
1000000 loops, best of 3: 784 ns per loop
```