Merge pull request #16 from yoni/master

Fixes based on CRAN maintainers' comments
Use correct suffix for CRAN binaries

If type = mac.binary, use .tgz
If type = win.binary, use .zip

Tests added in test-install-version

Passes when running R CMD check --as-cran

Note: other tests throw an error:

In untar(src, exdir = target, compressed = "gzip") :
    '/usr/bin/gnutar -xf '/var/folders/8q/pd5448hs7n92sy7d25lpnw600000gp/T//Rtmpx7aHMa/tempdisagg_0.13.tar.gz' -C '/var/folders/8q/pd5448hs7n92sy7d25lpnw600000gp/T//Rtmpx7aHMa/devtools337d2b2faa1c'' returned error code 127

 This error exists for me on the pristine copy from GitHub.

 Haven't debugged this error -- I'm assuming that R CMD check --as-cran passes on maintainer's system
DRY up the testing for package source type checks. Refs #18
Update docs, news, and version.
Fix error in extracting archive versions

Version number was not being properly extracted from the archive.

Archive URL was not updated with suffix based on type.
Force archive to be installed from source

CRAN archives do not have binaries, just source.
Merge branch 'master' of github.com:KentonWhite/rbundler
Merge branch 'master' of github.com:yoni/rbundler
Introduce Travis CI support. refs #22