Clarified "distribution" guidance

Addressing Issue #16.
Fixed "theme" naming error

The Category field guidance incorrectly referred to the metadata name as "category" when it should be "theme."
Edited schema to express temporal ranges in ISO 8601 format
Edited accrualPeriodicity to align with Dublin Core acceptable values
added Esri Geoportal Server, an open source catalog

licensed under Apache 2.0.
fixed all bulleted lists, some links and spaces in content
added nicer formatting to footnotes with link to anchor at bottom of page
Mass changes to schema
Updated metadata rationale
Format adheres to MIME types
All date fields now ISO 8601 compliant
Update distribution cardinality

Technically there should never be one more than one distribution array for a dataset.
Clarified temporal guidance
Added Distribution to top table of optional fields, fixed ABC order
Clarified that URL values should be strings.
Updated format to take MIME Type

Closes #117
Clarified temporal usage notes.

Taken from #116. Thanks Sean!
Fixed accrualPeriodicity example

Thanks Sean! Taken from #116
Updated language guidance

Thanks Sean! This incorporates #100
Added PrimaryITInvestmentUII

Closes #91