add support for unit testing from building package (including at GitHub)

Brian,
I am testing most (all?) of my R packages via [Travis](https://travis-ci.org/eddelbuettel), and given that Travis relies on Ubuntu, I figured it would useful to have it test against an actual redis server.  As I noticed that you already had RUnit tests, I just did the minimal additions to have these tickled -- if and only if an environment variable (currently:  `RunRRedisTests`) is loaded with the right value (currently: `yes`).  So CRAN builds would not get this, heck your own builds would not get it unless you set the variable.
But GitHub commits now trigger checks.  I missed the need RUnit at first, [but trial 2 succeeded](https://travis-ci.org/eddelbuettel/rredis/builds/16441880).
Feel free to discard the Pull Request if you find it too invasive, and/or hit me up on Google Chat / email if you have general questions regarding Travis.
Cheers,  Dirk