Add Riak CS multipart upload overview document
Add "active" to be explicit that it is not necessary to start and complete all remaining part uploads before an abort
Shorten some paragraphs, remove some redundant statements, other small reorgs to simplify for novices such as myself
Kelly ftw
Add a couple of sentences to describe some reasons for useing mp uploads
Some minor updates based on review feedback
Merge pull request #286 from basho/klm-mp-upload-info

Add Riak CS multipart upload overview document