Fix the build under the new testthat version. Fixes #26

* Migrate to new recommended test file structure.
* Fix testing of `.libPaths()` update. `expect_match` has been replaced
with an equivalent `expect_true`
Merge pull request #27 from yoni/testthat-upgrade

Testthat upgrade and prepare for release.
Add build status to the README

Also make some general improvements to sections.
README update

Make the build status image link to the Travis project instead of the image.
Conditional testing

Temporary resolution for https://github.com/opower/rbundler/issues/31 to skip tests on CRAN.