Add travis-ci support.

This adds support to automatically check httr with
[travis-ci](http://travis-ci.org), based on my prototype r-travis package.

I also had an overactive emacs setting strip trailing whitespace in
`README.md` -- but c'mon, who likes trailing whitespace? `;)`
Merge pull request #51 from craigcitro/travis

Add travis-ci support.
Add default user agent
Add default user agent the right way :/
Update to latest roxygen
Post needs to be loaded before patch
Make http_status more flexible
Classed http errors. Fixes #53
Import methods.
Closes #55. Closes #52.
Fix patch docs
Fix broken doc link
Automatic parsers for csv and tsv. Fixes #49
Fix typo and update mime types
Better parsing for empty query params.
Don't try converting query params. Keep as strings
Better escaping for query param names
Improve content() docs. Closes #50