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. 
Ok, you know the use-case better than I do.

On (Nov-24-14| 4:04), Antonio Gonzalez wrote:
> > @@ -1636,6 +1639,111 @@ def study_id(self):
> >              raise QiitaDBError("No studies found associated with prep "
> >                                 "template ID %d" % self._id)
> >  
> > +    def create_qiime_map(self, prep_template_fp):
> 
> OK on the name change but I do not agree on the prep id as this is a class method of PrepTemplate so you could simply do self.id. However, you can have several fps and it can be extremely tricky to know which one was just added so passing the fp solves this. 
> 
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> https://github.com/biocore/qiita/pull/678/files#r20785682