Cleanup and improvement of unit tests

Hello All,
I spent some time on the unit tests of RcppEigen today. This patch is basically some cleanup, and I also added a number of missing tests that could be included in this package. Below is a list:
- `as()` for mapped sparse matrices
- transform of 2D array (the C++ was already there, but the R code was missing)
- `wrap()` for row vectors
- `as()` for `unsigned int` type

Also, I noticed that the file `inst/unitTests/runit.RcppEigen.R` is an old one (updated 2 years ago) and the tests in it have been re-coded in `inst/unitTests/runit.wrap.R` and `inst/unitTests/runit.sparse.R` more recently, so probably we could delete it (not in this patch). It will also save some time in the unit testing since currently some equivalent/similar tests are conducted twice.