Dev release of 7.x-3.x.
Changes from 6.x-2.x:
- Module has been broken up into a base ESI module, and submodules for block/panels/context integration.
- No longer uses the theme-layer to render ESI tags (blocks and context makes use of hook_page_build_alter(); panels implements a custom rendering pipeline).
- New user-context API, to simplify adding arbitrary contexts (such as content which changes according to geographic location, user's age, or any arbitrary grouping).