Added docs for temporal model

Updates temporal docs to reflect change to dual timestamps

Adds documentation for delete functionality

Updates timestamp

Adds documentation for overwrite()

Adds more indepth explanation of temporal to introdction

Adds purge() documentation

Fixes typos and adds a clarafaction on cascade delete about how relations are handled.
Merge pull request #493 from stevewest/temporal

Added docs for temporal model
Added documentation for Lang::get_lang function
Changed lowercase doctype to uppercase DOCTYPE

While per HTML5 specification `DOCTYPE` should be parsed `ASCII case-insensitive` some (wrongly implemented) HTML parsers don't recognize it in lowercase. Probably because W3C given example is `<!DOCTYPE html>`
Merge pull request #494 from davispuh/patch-1

Added documentation for Lang::get_lang function
clarified how to enable the parser package