Added behavior to consider anchor tags when slicing, Fixes #87. 

Expander will now bother to check to see if the tag it's adding a read-more link into is an anchor tag (which would break the read-more), and will now avoid it.
 
Added test to check if expander places a read more inside an anchor link. 

Thanks to @kswedberg for getting me on the "right tree" to bark up.