Regenerate gemspec for version 2.15.5

Merge branch 'master' of github.com:waynerobinson/xeroizer

Bump Gemspec Version

Lock to OAuth 0.4.5

Corrects some issues with signatures being invalid for PUT requests.

Bring Version Numbers Inline with Xero API Version

Xeroizer's version numbers will be kept inline with Xero's
own API version numbers to make it easier to use Bundler.

The current Xero API version is 2.15 and the current Xeroizer
version is 2.15.0. Xeroizer patches will only modify the last
number in this version string.

Specifying `gem xeroizer, '~> 2.15'` in your `Gemfile` will
keep your bundle aligned with v2.15 of the Xero API.

Fix Ruby 1.9 Issues with Block Parameters

Patches for Today's Release:

* manual journal URL attribute should be 'Url'
* add show_on_cash_basis_reports to manual journal

Update API with Xero 2012-04-17 V2.15

Small Fixes Including Missing Attr & Summary Data

Bump to Version 0.4.3

Adds Address#attention_to Attribute