Added a __repr__ function for lightcurve and overrode default OrderedDict __repr__

  - I added a __repr__ function to lightcurve to display some summary information about a constructed lightcurve on request, similar to Map.

  - I also added (probably badly) an override in sunpy.util.odict to the OrderedDict __repr__ method to produce a more human-readable output, as it's very messy currently. This is noticeable when querying lightcurve metadata, for example.

I suspect that the second item involved some very bad python, so I'd appreciate any advice on how to do this properly! Or maybe it should be dealt with in AstroPy?