Add support for multiple multipart parts in a single call.

addMultipartParts

https://github.com/koush/ion/issues/231
Merge branch 'master' of https://github.com/koush/ion

# By Koushik Dutta
# Via Koushik Dutta
* 'master' of https://github.com/koush/ion:
  Add support for multiple multipart parts in a single call.