Add initial bash completion script

The script can be used to auto-complete commands, image names and
container ids from within a bash prompt.

This partially resolves #336.
Add description and usage information
Missed -entrypoint from 'docker run' options
Clarified bash completion limitations
Still missed -entrypoint from 'docker run'
docs/postgresql: PostgreSQL service on Docker example
fix/docs: ubuntu instead of base, note about root-only
Fixed typos
Merge pull request #1488 from KSid/336-bash-completion

* Contrib: bash completion script
Merge pull request #1504 from zaiste/docs/postgresql-service

docs/postgresql: PostgreSQL service on Docker example
API, issue 1471: Allow users belonging to the docker group to use the docker client
Merge pull request #1508 from pborreli/typos

Fixed typos
Merge pull request #1485 from dotcloud/1471-unixsocket-group

* Runtime: API, issue 1471: Use groups for socket permissions