Adds scaffolding for child theme (based on Twenty Twelve by default)

Apart from creating an empty underscore theme, it would be cool to be
able to create an empty child theme. I've implemented a very basic
command for this. It created a theme folder and style.css which imports
parent theme's styles
Regenerate Man
Added label to tax and cpt
Only preg_replace default $slug
add 'history' command
add man page for `wp shell`
compute the history file path at the beginning

this avoids problems if the user calls chdir()
Merge pull request #373 from wp-cli/shell-history

shell: retrieve history
remove --str flag from db subcommands

The --str flag assumes that all the information will be passed as CLI
arguments, but that's no longer the case since we started using the
MYSQL_PWD env variable.

And we might want to start using the PDO extension in the future,
instead of the `mysql` binary.

Also introduce run_mysql_query() utility.
create tests database if it doesn't exist
update db man page
Merge pull request #374 from wp-cli/tests-db

`wp core init-tests` should create the database
Added label scaffold behat test
Reorder tests
Added `post get` command.
Removed feature tag
Merge pull request #365 from linepress/scaffold-label

Scaffold cpt / tax with label
add --defaults-file=/dev/null to mysql commands
use run_mysql_query() in functional tests too
Removed the <filename> argument from `post get`.