Use process.cwd() instead of process.env.PWD

Windows Azure doesn't set process.env.PWD, but a simple string
concatenation using process.cwd() works just fine. I'm not using
path.join() here because on Windows systems the path seperator
character is backslash ("\") instead of slash ("/"):
http://en.wikipedia.org/wiki/Path_%28computing%29

Without this fix you'll see the following exception when running on
Windows Azure:

Tue Aug 27 2013 22:03:06 GMT+0000 (Coordinated Universal Time): Unaught
exception: TypeError: Arguments to path.join must be strings
    at f (path.js:204:15)
    at Object.filter (native)
    at Object.exports.join (path.js:209:40)
    at Object.<anonymous>
(\\100.68.130.79\volume-24-default\2c3d84fd3e95e8aaaaec\9d446c52519340f4
9df1c437ee86f24c\site\wwwroot\node_modules\static-expiry\index.js:27:17)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function._load
(\\100.68.130.79\volume-24-default\2c3d84fd3e95e8aaaaec\9d446c52519340f4
9df1c437ee86f24c\site\wwwroot\node_modules\newrelic\lib\shimmer.js:151:3
8)
    at Module.require (module.js:364:17)

I worked around the issue by adding:
process.env.PWD = process.cwd();

before the require statement:
var expiry = require('static-expiry');
Don't use path.join() to build URLs

On Windows systems the path seperator
character is backslash ("\") instead of slash ("/"):
http://en.wikipedia.org/wiki/Path_%28computing%29

This causes problems with URLs that end up rendered to the page as
"\fixtures\ec5cf137182e7c52d2a35d628d36f4e4-styles.css" which the
browser and/or server happily interpret as
"/fixtures/ec5cf137182e7c52d2a35d628d36f4e4-styles.css" which doesn't
belong in expiry's cache.

This commit fixes prefile, postfile, and path URL generation to work on
Windows Azure. The query URL generation was already working on Windows
Azure since it doesn't rely on path.join().

Also, path.dirname() appends a trailing slash ("/") character on *nix
systems but appends no path separator character on Windows systems. We
trim off any trailing slashes so they can be added consistently on all
platforms.

I made sure the unit tests are passing with these changes as well as
tested my Node.js app running on Windows Azure.
Merge pull request #5 from freshlogic/master

A couple small but important fixes for Azure