Added migration create command for creating empty migration classes

[motin]

Fixes #832

For semantic reasons, I chose to implement it as a separate command instead of an "--empty" command to migration:diff.

Usage and verbose output:

# vendor/bin/propel migration:create -vvv
"/app/dna/generated-migrations/PropelMigration_1449320135.php" file successfully created.
Now add SQL statements and data migration code as necessary.
Once the migration class is valid, call the "migrate" task to execute it.

[marcj]

Very cool, thanks! We should also add this to PropelBundle: propelorm/PropelBundle#372