ErasureCode: doc updates

* Update to the current state of the ghobject implementaiton and the fact
  that they encode the shard_t Although the pool also contains the shard
  id, it is less relevant to understand the implementation.

* Update with the erasure code plugin infrastructure and the example
  plugin now in master.

* Move jerasure to a separate page to be expanded and link it from the
  toc

* Kill the partial read and writes notes as it will probably not be
  implemented in the near future. Kill some of the notes because they
  are no longer relevant.

* Add a definition for "chunk rank"

* Reword, update schemas, fix typos.

Signed-off-by: Loic Dachary <loic@dachary.org>
ErasureCode: update PGBackend description

Based on a dialog with Sam ( as published at http://dachary.org/?p=2320 ).

* Remove PGBackend-h.rst because PGBackend.h is now in master.

* Fix typos caught by ispell

* Update recovery links to point to PGBackend recover methods

* Workaround formating warning
  developer_notes.rst:3: WARNING: Duplicate explicit target name:
  "erasurecodepluginexample" which should be legitimate.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #620 from dachary/wip-erasure-doc

ErasureCode: doc updates