add codecov badge

Fix confusing typo in example

change travis file to use newer R lang setup

Cran fix

- Kurt Hornik requested removing some top level files. Those are now in .RBuildignore
- Rebuilt the package under new roxygen/devtools. This required some changes to roxygen tags. Replaced @S3method with @export.

All checks pass except for this note:

* checking sizes of PDF files under ‘inst/doc’ ... NOTE

Now namespace is correct with Imports, updated news file. Will push to CRAN once I switch everything over to httr.
Signed-off-by: Karthik Ram <karthik.ram@gmail.com>

Signed-off-by: Karthik Ram <karthik.ram@gmail.com>

Signed-off-by: Karthik Ram <karthik.ram@gmail.com>

All depends have now been moved to Imports. I'll shortly switch everything over from legacy RCurl to httr as well but for now no package functionality has been broken.
Signed-off-by: Karthik Ram <karthik.ram@gmail.com>