Fix issue with Alter Column for Oracle

Alter Column defaulted text to "Not Null" if the column was not nullable
(the default)
Alter.Column("SystemName")
.OnTable(AlertDefs)
.AsString(255);
would result in
ALTER TABLE stAlertDefs MODIFY SystemName NVARCHAR2(255) NOT NULL

This is not allowed by Oracle because the column is already not null.

Solution was to make IsNullable a Nullable<bool> and only add "Not Null"
to the alter if .NotNullable() was specified.  This should only affect
Oracle.
don't check if schema exists for VersionTableMetaData if not using Schema
Default Oracle Identifier quoting is no quotes

Still need to add a context property to specify which quoter to use
add missing quoter class
Added provider switch and made oracle quoting optional
Move OracleProcessorFactoryTests to Integration
PostgresGenerator can handle nullable IsNullable

Oracle change made IsNullable to nullable so the PostgresColumn and
PostgresGenerator have to handle when IsNullable on a column is not
set.