Adding support for back-of-the-book index

[Jan Tosovsky <j....@email.cz>]

Dear All,

I plan to dedicate part of my spare time to implementing the index support.
Considering my other duties and the fact I still learn the FOP internals do
not expect results in next weeks, but rather months.

I've just created Jira ticket for this:
https://issues.apache.org/jira/browse/FOP-2851

I am just announcing this to not duplicate our work in case someone else
would have same idea :-)

For now I have some ideas what needs to be covered (reusing the knowledge
from change bars support), but I suppose in later stages I'll appreciate
some help.

Jan