Changed DESCRIPTION Depends to R(>=3.0.2)

Using tools::assertError in tests/arguments.R depends on R(>=3.0.2)
Updated the test code layout

All test code have been changed to have the following layout in order to
give better information in case of an error.

a2.exp <- c(1,2,3,4,5)
filename <- tempfile(fileext = ".mat")
write.mat(list(a=a2.exp), filename=filename, compression=FALSE, version='MAT5')
a2.obs <- read.mat(filename)[['a']]
unlink(filename)
str(a2.obs)
stopifnot(identical(a2.obs, a2.exp))

if(rmatio:::have.zlib()) {
    filename <- tempfile(fileext = ".mat")
    write.mat(list(a=a2.exp), filename=filename, compression=TRUE, version='MAT5')
    a2.zlib.obs <- read.mat(filename)[['a']]
    unlink(filename)
    str(a2.zlib.obs)
    stopifnot(identical(a2.zlib.obs, a2.exp))
}