Documentation for the upcoming 0.8.1 release

Hey guys,

I took a stab at updating the docs for the 0.8.1 release. In particular, I
added a section on log compaction:
http://kafka.apache.org/081/documentation.html#compaction

I also updated the configs. This is all under 081, I will flip this over to
be the main documentation when 0.8.1 is released but you can see it now at
http://kafka.apache.org/081/documentation.html#compaction

Any comments on improvements we could make would be much appreciated!

-Jay