implement '@when' docblock tag

Works on both commands and subcommands.
calculate path for early commands only once
show error on invalid json. closes #531
Merge pull request #453 from wp-cli/doctag-when

Command phpdoc or other flag to allow third-party commands to run before WordPress is present
add optional 'before_invoke' parameter to WP_CLI::add_command()

this avoids repeating the command name and makes it clearer when the
callback is fired
Support Subdomains
Merge pull request #532 from MiteshShah/patch-1

Support Subdomains
core install-network: flags don't need a value; update man page

see #532
search-replace: always do a SHOW TABLES query
Merge pull request #534 from wp-cli/search-replace-tables

search-replace: query tables instead of relying on what's in wpdb
option update: show success message even if the value has not changed

see #531
post create: balance brackets
behat: add test for `post create` reading content from STDIN
--info: make WP-CLI root be the root folder, not php/
remove support for Phar archive
Merge pull request #439 from wp-cli/phar-away

Remove support for Phar archives
define WP_CLI_ROOT to be the root directory, with no trailing slash
Merge pull request #537 from wp-cli/cli-root

Fix WP_CLI_ROOT
colorize plugin/theme status legend. see #517
fix man page for `wp core is-installed`