Backend: Removed unnecessary check for NULL pointer before delete.

This addresses issue #64, (mildly) reduces the amount of code, and
simplifies certain automated testing methodologies.

Reported by
GCC 4.8: Fixed all warnings obtained with -Wall -pedantic -Wextra
Doc: Updated doxygen comment on matrix class. Both row- and column-major layouts supported.

Reported-by: Pushkar Ratnalikar <pratnali@umail.iu.edu> via viennacl-support.