Add mu-plugin to fix Nginx permalink issue

Because WordPress cannot correctly identify whether rewrite
capabilities are enabled on this server setup, "index.php" is getting
injected into the top level of the permalink structure (e.g.
http://www.mysite.com/index.php/sample-post/). Hooking into WP's
`got_rewrite` filter to explicitly specify that rewrites are available
removes the issue, as explained in the closing comments for the
core Trac ticket 13201 and demonstrated in the code for rtCamp's
"Nginx Helper" plugin.

In the interest of clarify, this plugin uses the simplest possible
code (i.e., no plugin Class boilerplate).

References:

Trac ticket for Nginx permalink issues
http://core.trac.wordpress.org/ticket/13021#comment:4

Nginx Helper plugin
http://wordpress.org/plugins/nginx-helper/

Fixes #85