Support added for Play! DB Evolutions.

Note that evolutions are disabled by default. Parameter activator_apply_evolutions must be
set to true in deploy.rb to activate them.
Support for extended configuration options and production ready README.

* Jvm args can now be set in deploy.rb or using extra params.
* NodeJS assets compilation can now be disabled.
* README updated with "How it works", "Tasks" and "Configuration" sections.