Add public isExpired() method, and skipRemove/allowExpired get() parameters

I've added a little extra functionality for my own usage of lscache.  I use lscache to cache AJAX responses to reduce calls from mobile apps.  But, the problem I ran into was once the cache expired and I got a null response from get(), I launch another ajax request to refresh the data.  However, if that call fails (no wifi connection, server error, etc.), there's no longer any cached/expired data available to use as a fallback.  With the ability to call get() but skip the automatic removal, I can preserve the expired data as a fallback if needed.  I have a jQuery plugin using the modified version of lscache here: https://github.com/brophdawg11/jquery.ajax.lscache