Fix support java migrations with index

[rPraml]

Hello Rob,
with #91 the ebean-migration does not scan the classpath for java migrations. There are currently 2 issues

1. When Ebean generates migrations, it does not add java migrations to the idx file (PR will follow)
2. Ebean-migration does not support java migrations in the idx file.

This PR adds java-migration support for idx files.
Furthermore, this change may make #90 obsolete (the java class to use is defined in the idx file now)

[rbygrave]

Well, as I see it the right way to find and initiate Java Migrations long term is to use ServiceLoader. The way that Java Migrations work currently as I see it is pretty horrible relative to using a ServiceLoader [with class path scanning and reflection used to construct instances etc].

In that sense, this PR keeps the current Java Migrations working with the index file ... but for me this would only be a temporary solution and that the proper solution is to actually change to use ServiceLoader.

Yes, currently applications that use the existing Java Migrations API can't use the index file. I need to get a sense of what the long term solution looks like using ServiceLoader and then see what that would mean for existing Java migrations etc.

[rPraml]

Hello Rob,

then wait with merging this PR.
I have some ideas how to implement this, as I do not like the hacks that we have to do in ebean.