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.
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