Build and deploy swiftdoc using jazzy

[gburd]

• Move from using ghp-import to using a travis deploy stage to commit documentation updates.
• Add Jazzy installation and swiftdoc generation to document building script.
• Commit generated documentation to a new branch gh-pages-api-docs. The existing gh-pages branch was incorrectly formatted and it was easier to just create a new branch.
• Remove existing cargo-doc script.
• Ensure build phase only executes on the master branch.

TODO: Point GithubPages to gh-pages-api-docs branch rather than master. This is done directly in settings and so will not form part of this PR. It needs to be done once this patch has landed.