replace mongokit's find_one with fetch_one for embedded schemadocs

if a Document object containing an embedded SchemaDocument is
retrieved with find* instead of fetch* methods, the embedded doc is
recreated as a dict, not as a SchemaDocument.
replace all Document.find* calls with Document.fetch* calls.
Merge branch 'develop' of https://github.com/mobyle2/mobyle2.web into develop