Added ability to register for receiving updated x, y coordinates from the 
matplotlib viewer when its circle is moved.
Introduced ImageReading class to associate a reading with an image after
it has been downloaded.
Merge branch 'master' into reals_proc
Added cutout to logger debug statement.
ImageReading now exposes x, y, ra and dec values.  The x and y values 
can be updated, in which case the ra and dec values will be lazily refreshed
with a call to wcs.xy2sky (call still to be implemented).
Added parse_cd and parse_pv functions to wcs module so it can extract
the values from astropy FITS headers.
Added parse_order_fit function to wcs module.
Implemented updating RA and DEC for an ImageReading object when the 
operator has adjusted the x and/or y values.
Moved get_observed_magnitude method from DownloadedFitsImage to
ImageReading.
The UIModel now provides the updateable RA/DEC values instead of the 
static ones read from the input file.
Moved calculation of source point from UIModel to ImageReading.
Added get_astrom_header and get_fits_header methods to UIModel
and refactored.
Added method to coordinate converters to obtain a second converter 
which makes the transformation back from the original's output 
coordinate system to its input coordinate system.
Added method to DownloadedFitsImage for converting from pixel 
coordinates to the observed coordinates.
Made distinction between pixel source location and observed source 
location in the model.  Pixel source location is the coordinates of the 
source in the cutout image, while observed source location is in the 
coordinates of the original image (as used in the astrom file).

Calculating observed magnitude with daophot uses the pixel source 
location whereas calculating RA/DEC with xy2sky uses the observed source
location.
Fixed access of source pixel location from model by controller.
Hooked up controller to receive repositioning events from matplotlib
viewer.  The model now has an update current source location method
which the controller then calls.  

At this point the user is now able to update the model by interacting with 
the image viewer.  Adjusting the circle around the source changes the
observed magnitude calculated, for example.
Merge branch 'master' into reals_proc
Added hacky and not robust way of making keyboard shortcuts work 
after clicking in image viewer.  A better way will be needed.
Made process of giving focus back the main panel after interacting with
viewer slightly less brittle.  Still seems like there should be a better way.