Add before_put_image_json signal

Add before_put_image_json signal that allows receivers to short-circuit
the put if they return any non-None value. This can be useful for things
such as enforcing quotas, e.g. by having a custom extension connect to
this signal, check the layer's Size, and returning something to the
effect that the put was denied due to quota.

Signed-off-by: Andy Goldstein <agoldste@redhat.com>
Add standalone as default example
Merge pull request #607 from ncdc/new-put-json-signal

Add before_put_image_json signal
Bump and pypi publish core

Docker-DCO-1.1-Signed-off-by: Olivier Gambier <olivier@docker.com> (github: dmp42)
update Dockerfile to follow conventions

change instructions to be uppercase
change ADD to COPY
change cmd exec docker-registry to CMD ["docker-registry"]
Merge pull request #620 from unclejack/dockerfile_changes

update Dockerfile to follow conventions
fix forgotten lowercase cmd
Merge pull request #621 from unclejack/uppercase_cmd

fix forgotten lowercase cmd
Config template: add boto_calling_format S3 option

Using the "ordinary" calling format can be useful when using
a bucket whose name contains periods (otherwise the AWS SSL
certificate may be refused because it is several subdomains
deep).
Merge pull request #625 from krallin/aws-calling-format

Config template: add boto_calling_format S3 option
Correcting typos in the {ADVANCED, CONTRIBUTING, README}.md files.
Merge pull request #628 from krook/fix-markdown-typoes

Correcting typos in the {ADVANCED, CONTRIBUTING, README}.md files.
Do not use conflicting module names for bugsnag
Do not use conflicting module names for newrelic
Alphabetical ordering of imports
Merge pull request #638 from gregburek/bugsnag

Do not use conflicting module names for bugsnag or newrelic
README.md: Shift Index discussion to the external-backend section

We use Index for third-party search engines.  For the 'sqlalchemy'
special case, we use SQLAlchemyIndex.
README.md: Document --preload for the SQLAlchemy search index
avoid using same variable twice in the same function
Merge pull request #659 from misho-kr/master

avoid using a variable twice in the same function