replaced expensive with extensive in the Readme.md

[Joshua-Fredrickson]

in the following section I replaced expensive with extensive for improved comprehension.

Working with state

When working with the pushState API, and page.js you may optionally provide state objects available when the user navigates the history.

For example if you had a photo application and you performed a relatively extensive search to populate a list of images, normally when a user clicks "back" in the browser the route would be invoked and the query would be made yet-again.

An example implementation might look as follows:

[coveralls]

Coverage remained the same at 91.429% when pulling 06db2b63364f447fd9db321b245ae61f710f9a3e on Joshua-Fredrickson:joshuaFredrickson into 1bc93692b0924b686e2207a66e2e73a85e1ab058 on visionmedia:master.

[coveralls]

Coverage remained the same at 91.429% when pulling 06db2b63364f447fd9db321b245ae61f710f9a3e on Joshua-Fredrickson:joshuaFredrickson into 1bc93692b0924b686e2207a66e2e73a85e1ab058 on visionmedia:master.

[matthewp]

Thanks.