Add travis-ci config file.

[#24]
Run default grunt tasks on TravisCI.

[Finishes #24]
Merge branch 'master' into travis-ci-issue-24
Add ability to override audit rule severity in AuditConfiguration, add test; add extra test for BadAriaRole too
Merge pull request #43 from alice/severity

Add ability to override audit rule severity in AuditConfiguration
Merge pull request #38 from ckundo/travis-ci-issue-24

Travis ci configuration
Update README to give a url for including axs_testing.js
Merge pull request #49 from ckundo/document-releases

Update README to give a url for including axs_testing.js