Merge pull request #28 from davetoxa/4.0

Правки 02-active-record-validations-and-callbacks
Merge pull request #29 from davetoxa/4.0

Правки 03-active-record-associations
Update source/18-caching-with-rails-an-overview/0-basic-caching.md

Update information about gzip and correct a mistake.
Merge pull request #32 from ouyoutouchmytralala/patch-1

Update source/18-caching-with-rails-an-overview/0-basic-caching.md
Merge pull request #31 from davetoxa/4.0

Update active_record_querying, active_record_validations, layouts_and_rendering
update capistrano #34
update debugging guide
performance guide updated
update configuring guide
update command line guide
update caching guide
assets guide updated
rewrite release notes for 4.0
update callbacks guide
upgrading guide added
update engine guide
update rack guide
small fix on main page
remove discussion view specs
add numeration for headers