Adding qiime map file

This adds the creation of the qiime mapping file (combination of prep/sample template) at the when the prep template is added. Note that the code is ready for review, it tests pass, but we should merge https://github.com/biocore/qiita/pull/677 first. 
Ooops never mind, I noticed now that we are under the PrepTemplate... This logic is a bit weird, but I cannot see a better way of solving it, so good as it is.