FITSFigure.recenter now uses radius in pix, not deg anymore

Previously `FITSFigure.recenter(x, y, radius)` took `radius` in deg ... now it seems it takes pix. To reproduce:
```
wget http://heasarc.gsfc.nasa.gov/docs/rosat/survey/sxrb/rass_c.fits
```
```python
from aplpy import FITSFigure
f = FITSFigure('rass_c.fits')
f.recenter(0, 0, 5)
f.save('image.png')
```
There's no error that ```pixel_scale = wcs_util.pixel_scale(self._wcs)``` is failing.

Also the docstring should mention if it takes `deg` or `pix`:
```
        '''
        Center the image on a given position and with a given radius.

        Either the radius or width/heigh arguments should be specified.

        Parameters
        ----------

        x, y : float
            Coordinates to center on

        radius : float, optional
            Radius of the region to view. This produces a square plot.

        width : float, optional
            Width of the region to view. This should be given in
            conjunction with the height argument.

        height : float, optional
            Height of the region to view. This should be given in
            conjunction with the width argument.
        '''
```
![image](https://f.cloud.github.com/assets/852409/555752/676d49c8-c3c6-11e2-814c-b0acc888b955.png)