Allow scaling of dark frames

[mwcraig]

The current call to ccdproc is:

        return ccdproc.subtract_dark(ccd, master,
                                     exposure_time='exposure',
                                     exposure_unit=u.second)

This is inflexible in a few ways:

• Requires the keyword to be named exposure and the units to be second.
• Assumes there is an exact match for the exposure time.
• Does not allow scaling.