Use array instead of Hash to keep the registered filters

1.8.7 compatibility fix

In Ruby 1.8.7, Hash does not preserve insertion ordering as Array does.
This could cause a problem when registering filters which depend on others and
the registration order is important.

So, the @@filters variable was changed to array where the order of the filters is
the same as the insertion order.
Merge pull request #135 from astathopoulos/preserve_filters_ordering

Use array instead of Hash to keep the registered filters