Return nothing if FITS file is not available

Cleaner/easier to read than throwing all the time, I think.
Tweak layout so labels don't fall outside image
Fix formatting of lightcurve graph

This should cure the overlapping labels problem.

I've removed the argument to set T0 (the origin of the time axis) for this
plot, since it didn't seem to be used anywhere. Happy to resurrect it if
anybody has a use case.
Fix typo
Fix inconsistent capitalization
Restore T0 argument to transient_plot
Merge pull request #4 from jdswinbank/formatting

Formatting