#508 Enable reading FitsHeader from SCamp .head file

Reiko was trying to read in the WCS from a .head file produced from SCamp, and none of our WCS classes worked.  Mostly because a .head file is not something pyfits (or anything really) knows how to open.  However, as Gary pointed out, it's pretty easy to convert it into a pyfits header, so I have now enabled this with the syntax
```
header = galsim.FitsHeader('blah.head', text_file=True)
wcs = galsim.GSFitsWCS(header=header)
```
I also added this option to the FitsWCS function, so another way to do the same thing would be 
```
wcs = galsim.FitsWCS('blah.head', text_file=True)
```