Caching the Inverse of a Matrix

This commit addresses the following:
1. Introduces the makeCacheMatrix and cacheSolve implementation.
2. Documents the functions through code comments which use
[roxygen2](http://cran.r-project.org/web/packages/roxygen2/index.html)
so that can be auto-generated into rd.
3. Adds matching unit test for function to `tests/cachematrix.R` and
the UNITTESTS.md document explaining how to setup and execute.

Note: While writing the unit tests using the
[testthat](http://cran.r-project.org/web/packages/testthat/index.html)
library, I uncovered [issue
#197](https://github.com/hadley/testthat/issues/197). Subsequently I
provided a fix through [pull request
#196](https://github.com/hadley/testthat/pull/196) as well as a
temporary workaround for others to use until the issue is addressed
(this workaround is also used in the tests).