add contributing authors to book info, exports

Adds a 'Contributing Authors' field to Book Info. Conforms to EPUB spec:  http://www.idpf.org/epub/20/spec/OPF_2.0_latest.htm#Section2.2.6

In Book Info:

![screen shot 2014-08-29 at 1 14 55 pm](https://cloud.githubusercontent.com/assets/2048170/4095790/3d7c07d4-2fb9-11e4-94c6-1b72ed99afa7.png)

Export routines, microdata/metadata, footer fields have been addressed: 

![screen shot 2014-08-29 at 12 59 54 pm](https://cloud.githubusercontent.com/assets/2048170/4095834/76ca56d0-2fb9-11e4-9152-756a455e30b5.png)

![screen shot 2014-08-29 at 12 15 42 pm](https://cloud.githubusercontent.com/assets/2048170/4095836/7e335e62-2fb9-11e4-80d3-b741244531a4.png)

![screen shot 2014-08-29 at 12 15 23 pm](https://cloud.githubusercontent.com/assets/2048170/4095837/8344a64a-2fb9-11e4-98b1-8c44cb4b2e4d.png)