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."
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
Clarified date for issued/modified

This addresses #65.
Alphabetized guidance section
Added bureauCode
Added programOffice
Renamed person to contactPoint

We may as well go all-in on DCAT alignment while making other changes. This closes #133.