[AutoScriptMigration] AutoScriptMigration implementation

it is new migration type, that allows to speed up the creation of
migration containing embedded scripts through conventions
[AutoScriptMigration][TEST] Test AutoScriptMigration Conventions
Postgres - Implement more of AlterColumnExpression

- modifying column to be nullable or not nullable
- alter default constraint
- delete default constraint

Closes #384
New schema query for DefaultValue

Checks if the a default value (constraint) exists for SqlServer,
Postgres and MySql. Uses the query in an integration test for
MigrationRunner.
SqlServer Alter Column Default Value fix

When altering a column's default value and setting it one of the
SystemMethods it used to returned the name of the enum rather than look
up the correct function. Closes #398 and closes #394.
Merge pull request #411 from daniellee/issue398-AlterColumnSystemMethods

SqlServer Alter Column Default Value fix
Add more tests for Postgres Alter Default Value
Merge pull request #410 from daniellee/issue384-postgresnotnull

Fix alter column for Postgres
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.
Merge pull request #407 from spaccabit/AutoScriptMigration

Auto script migration
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