Fix 'rails generate' with path corrections

The 'rails generate reportable_migration' command was looking in lib, but the actual requires were looking in the top-level directory. This moves generators back to lib and fixes the require path in railties.rb.

Prior to this, none of the generators would be available under Rails 3.2.11.

Afterwards:

    $ rails generate 
    Usage: rails generate GENERATOR [args] [options]
    [ snip ]

    ReportableJqueryFlotAssets:
      reportable_jquery_flot_assets

    ReportableMigration:
      reportable_migration

    ReportableRaphaelAssets:
      reportable_raphael_assets