Proposed changes for PrependMigrate

* Changes spacing on all the assembly to be more readable

* Moves prependmigrate.rb to prepend_migrate.rb for consistency with the module's name (a module/class with camel-cased name FooBar should be in a snake-cased file foo_bar.rb)

* Puts PrependMigrate under the Payload::Windows namespace

* Moves the ```#generate``` method back to Payload::Windows which I think makes it a little clearer how the payload gets put together.