Blog commands

I've added three new `wp blog` commands:

 * `wp blog info` outputs a simple informational overview of the current blog (site title, home URL, admin URL, admin email).
 * `wp blog home` launches the blog's homepage in the browser.
 * `wp blog admin` launches the blog's admin URL in the browser.

Let me know if you think these will be of use and I'll see about generating the man pages.