Adds Command for External Magento Connect Validation

This pull request adds an `extension:validate` command to `n98-magerun`.  This command provides **external** validation of a system's Magento Connect instance. 

Magento Connect has an ongoing problem with PHP file permissions — it's possible to install an extension, have Connect report the extension as installed, but behind the scenes Magento Connect has failed to write out a file due to permissions issues. These errors get swallowed by the `@` operator. 

This command uses the `download/cache.cfg` file to grab a list of packages installed via Magento Connect, and validates the file manifest.  It looks for both the existence of the file, and checks the file's hash.  By default  the list of Magento included packages is omitted, since many users don't use connect to manage their Magento versions.  More info in the help, suggestions for improvements/more tests prior to merging are more than welcome.