Custom address positioning and minor change in README

Thanks to @nickw108 for this one! This PR replaces #159 .

In Switzerland some C5 sized envelopes have their window on the right side instead of the left

![bildschirmfoto 2014-10-25 um 20 31 28](https://cloud.githubusercontent.com/assets/58966/4780730/39f96cf8-5c75-11e4-9def-ada184e099ab.png)

I therefore added a new config setting `Header Blocks Position` 

![configuration___system___magento_admin](https://cloud.githubusercontent.com/assets/58966/4780707/887c6f52-5c74-11e4-8f8e-a279748e71b7.png)

which allows you to switch the customer address block and the invoice data block.

![invoice2014-10-25_19-00-11_pdf_und_invoice2014-10-25_18-59-51_pdf](https://cloud.githubusercontent.com/assets/58966/4780708/9055c8fe-5c74-11e4-9447-122b57935a5f.png)