Merge pull request #9 from dmcglinn/master

Added text to the standard structure section
Update data_sharing_ms.md

Adds a period to the null values table.
added carriage returns to metadata section
added carriage returns to quality control section
note to self on adding MR info to metadata section
added some refs to the metadata MR section
added software info, citations
linked to webpages for metadata standards
Edited the standard data structure section by adding 5 specific guidelines, added a table to illustrate cross-tabs, and deleted material on unique row ids.