Added help tooltips to home page

Implements #69.  Added help text directly to "Services" and "Datasets"
 heading via tooltips.  Moved help text to separate include file in raw text,
 which both the main help and home page now include.

Also fixed capitalization error on
"NGDC Geoportal" and decapitalized "datasets" near end of help text.
Merge pull request #97 from benjwadams/add_help_tooltips_to_home_page

Added help tooltips to home page