Expose connection string in a migration

The connection string that is passed into the runner is now exposed as
property in the MigrationBase class.
TaskExecutor - Conn string set after load

If no connection string is set in the RunnerContext and
is instead loaded by ConfigStringManager then set the
Connection property so that it is passed onto the
MigrationRunner.
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.
Refactoring to use conn string from processor

Expose conn string in MigrationBase which is fetched from the Processor
class when MigrationRunner is executed and passed down to MigrationBase.
Merge branch 'issue240' of https://github.com/daniellee/fluentmigrator into daniellee-issue240
Merge pull request #414 of daniellee/issue240
Prefetch connectionstring to ensure security info is still present

Fix issue with Test, constructing with connection is not expected
Checked in IntegrationTestOptions by mistake
Clean up old generator test
Refactored existing tests
Renamed generator unit tests
Code cleanup for existing generator unit tests
Adds generator column unit tests
Adds generator constraint unit tests