add test for device-mapping of page-locked host memory
test mapping of host memory by direct CUDA calls

The device mapping of host memory works also with the Tesla C1060 if we make
use of direct calls to the CUDA library. This points at a problem in
cuda_wrapper.
host::vector: fix allocation flags

cudaHostMalloc() needs to be called with the flag cudaHostAllocMapped,
otherwise calling cudaHostGetDevicePointer() fails.

We should check whether this flag impacts the performance and whether there are
use cases of host::vector where the default mode is preferred.
host::vector test: purge mapped_host_memory_pure

The test case was useful to point out a problem in the allocation of
host::vector and is not needed anymore.
texture: replace private reference by pointer

A default initialisation of a reference is not possible without
potentially dangling objects. (Clang ≥ 3.3 and GCC ≥ 4.8 complain about
this.) The use of pointers is semantically equivalent and safer.
cuda_wrapper: disable warning for CUDA 5.5 headers

Clang ≥ 3.3 emits a warning for CUDA 5.5 headers:

    /usr/local/cuda-5.5/include/cuda_runtime.h:225:33: warning: function
    'cudaMallocHost' is not needed and will not be emitted
    [-Wunneeded-internal-declaration]