ENH: Add functions to summarize metadata stats
API: Change the name of the functions
ENH: Cast from defaultdict to dict
ENH/BUG: Change datetime objects to strings
TST: Add corresponding tests for util.py
DOC: Add docstrings for added functions in util.py
STY/ENH: Remove default dict and change names

It was pointed out that defaultdictionaries were not needed and that
some function names were rather confusing.
STY: Change variable names and use viewitems

viewitems is a py2/py3 compatible version of items for dictionaries.
Additionally some of the variable names were changed to be more
descriptive about the data they contain.
Merge pull request #265 from ElDeveloper/metadata-stats

Add metadata wrangling functions to qiita_ware