Fixed issue with partial xlim or ylim and facets

Because the earlier submitted patch was using plt.xlim() to get length
of the auto adjusted axes length, all axes in a facets plot would end up
with the same upper limit when setting only the lower limit and vice
versa. The same is was true for the y-limit.
Added unittest to check for bug fixed in last commit
Merge pull request #313 from arnfred/limits

Fixed issue with partial xlim or ylim and facets