Automatic link and image checking

This pull request adds a Gem called [HTML Proofer](https://github.com/gjtorikian/html-proofer) to the existing Travis CI test suite that verify that proposed changes do not prevent the site from building.

HTML Proofer checks the site to ensure that all images and links are valid, and that all images have alt attributes (508). No additional action is necessary on the part of the maintainers, however, when a pull request is submitted, Travis will verify all the links, and the pull request will indicate if the proposed changes will break any existing links, or includes any new, invalid links. This is true for both internal and external links.

Here's the [example output for this pull request](https://travis-ci.org/project-open-data/project-open-data.github.io/builds/17149054).

Running the new tests on the site as is, the following links are dead links:

```
Checking 351 external links...

Ran on 31 files!
./_site//glossary/index.html: internally linking to policy-memo/, which does not exist

./_site//api-basics/index.html: External link http://apievangelist.com/2011/01/26/history-of-apis-twitter/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/01/28/history-of-apis-amazon-e-commerce/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/01/30/history-of-apis-google-maps-api/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/03/11/history-of-apis-foursquare-api/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/03/11/history-of-apis-instagram-api/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/01/26/history-of-apis-ebay/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/01/28/history-of-apis-facebook-development-platform/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/01/28/history-of-apis-salesforce-com/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/03/12/history-of-apis-amazon-ec2/ failed: 404 No error

./_site//api-basics/index.html: External link http://apievangelist.com/2011/03/12/history-of-apis-amazon-s3/ failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/application_gallery.php failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/basic_auth.php failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/code_libraries.php failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/documentaton.php failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/error_response_codes.php failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/github.php failed: 404 No error

./_site//data-hubs/index.html: External link http://www.fas.usda.gov/data.asp failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/oauth.php failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/software_development_kits.php failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/terms_of_use__conditions.php failed: 404 No error

./_site//glossary/index.html: External link http://apievangelist.com/buildingblocks/usage_logs_analytics.php failed: 404 No error

./_site//faq/index.html: External link http://www.whitehouse.gov/sites/default/files/omb/memoranda/2013/m-13-09.pdf. failed: 404 No error

./_site//policy-docs/index.html: External link http://project-open-data.github.io/assets/docs/sample_data_call_procedure failed: 404 No error

./_site//index.html: External link http://www.data.gov/sites/default/files/attachments/Privacy%20and%20Security%20Checklist.pdf failed: 404 No error
```

//cc @kinlane, @gjtorikian