Added samples to sparse documentation

[mlloreda]

Samples added for:

array sparse(const dim_t nRows, const dim_t nCols,
                       const array values, const array rowIdx, const array colIdx,
                       const af::storage stype = AF_STORAGE_CSR);

array sparse(const dim_t nRows, const dim_t nCols, const dim_t nNZ,
                       const void* const values,
                       const int * const rowIdx, const int * const colIdx,
                       const dtype type = f32, const af::storage stype = AF_STORAGE_CSR,
                       const af::source src = afHost);

array sparse(const array dense, const af::storage stype = AF_STORAGE_CSR);

array dense(const array sparse);

Also made a few minor improvements to sparse backend and API code.

Addressed feedback