replace iconv with string.encode for ruby 1.9 support

First I want to say: awesome plugin, thanks for writing it!

I tried using it with ruby 1.9 but got errors related to Iconv. With all the encoding related changes in 1.9 it turns out the recommended way to do this [has changed](http://stackoverflow.com/questions/8148762/iconv-deprecation-warning-with-ruby-1-9-3/11287648#11287648).

The [other answer linked inside](http://stackoverflow.com/questions/7870636/equivalent-of-iconv-convutf-8-ignore-in-ruby-1-9-x/7870859#7870859) does express some concerns that string.encode does not work exactly like iconv, but the guy who wrote string.encode says that [the differences should be small enough that you probably don't need to worry about it](http://www.ruby-forum.com/topic/993446).

Anyway, I tested the change on my own jekyll site and everything seemed to work okay.