This is a mostly trivial change, but it's also more consistent--if the other unit conversions in this class are using abbreviations why shouldn't lightyear? Alternatively, both versions could be made available (i.e 'lightyear' and 'lyr', likewise 'pc' and 'parsec', etc.)
Added long-form and short-form versions of all the unit conversions.  Also added 'redshift' as a long-form for 'z'.  Not sure if that's really necessary though since I think 'z' is pretty well understood.  I'm also not sure how I feel about spelling out 'astronomical_units' but I put it in for completeness.
Merge pull request #627 from iguananaut/lightyear

Distance.lightyear -> Distance.lyr