Add NotNull attribute and support for extended error codes.

Fixes praeclarum/sqlite-net#187

Merged-by: Øystein Krog <oystein.krog@gmail.com>
Fix errors related to notnull (due to missing functions) in platform packages.
If a [MaxLength] attribute is not set on a string property, don't limit the size of the varchar

Merged-by: Øystein Krog <oystein.krog@gmail.com>
Remove dead code (leftover from previous commit which removed the default varchar limit)
Support for DateTimeOffset.

Merged-by: Øystein Krog <oystein.krog@gmail.com>
Fix SQL errors when using a blob as a search criterion

Merged-by: Øystein Krog <oystein.krog@gmail.com>
Add missing UpdateAllAsync method similar to InsertAllAsync.

Usage is the same as InsertAllAsync

Conflicts:
	src/SQLiteAsync.cs

Merged-by: Øystein Krog <oystein.krog@gmail.com>
Added 2 new CreateIndex() methods which take a string[] columnNames argument.

This is just a much much much cleaner way of providing an API that is meant
to index an array of columns in a table.

Also fixed the gross hack that CreateTable() was using in order to index
multiple columns by using the new API.

Merged-by: Øystein Krog <oystein.krog@gmail.com>