integrators (gpu): reduce memory access, fix bug with g_image

For most particles, the image vector is likely to remain unchanged in
one invocation of the kernel. Thus, reading and writing the image vector
in any case wastes bandwidth.

The change revealed a severe bug in the Andersen and Nosé-Hoover
thermostats: the image vector was never written back. The bug is
probably only relevant if tagged particle displacements over long times
are considered, e.g., in the computation of the MSD. In this case, the
unmodified dynamics (i.e., without thermostat) should be used.

Nevertheless, it is embarrassing that the bug was not detected by any of
the unit tests.

Results from the Lennard-Jones benchmark (before/after the patch) for
the first step of the velocity-Verlet integrator (64,000 particles):

    Tesla T10:  255 µs → 215 µs
    Tesla K20:   95 µs →  85 µs
libs: remove boost.log submodule

Upgrade to Boost.Log 2.2, this removes the dependency of this module in
libs/ and makes the manual building obsolete, resulting in a faster
process when building the whole project from scratch.

Note that this lifts the minimum required Boost version to 1.55.0.

Closes #274122
CMakeLists: fix issues with static linkage of Boost libraries

Store HALMD_USE_STATIC_LIBS as cache variable. Otherwise, subsequent
CMake calls would fall back to dynamic linkage.

The Boost.Log library depends on Boost.Filesystem and must be mentioned
first on the command line. The wrong order causes many linker errors:
"undefined reference to `boost::filesystem::something`".
packages.mk: update several packages

Update the following packages to their recent versions:

    Clang (3.4)
    GCC (4.9.0)
    Git (1.9.2)
    HALMD (1.0-alpha1)
    HDF5 (1.8.13)
    Lua (5.2.3)
    LuaJIT (2.0.3)
    luatrace (6150cfd: Feb 17, 2014)

Remove "GNU readline wrapper", which is not related to HALMD.

Fix output of "eval `make -f packages.mk env-PACKAGE`"

Update top-level targets.

The update follows git://git.colberg.org/packages.git, which is
maintained by Peter Colberg and distributed under the MIT license.
test/…/two_particles: fix script errors, make test pass

Due to the constantly failing tests, necessary changes in the interface
have not been detected.

Convert the enforced error message into an author warning, which can be
disabled by passing --Wno-dev to CMake.
Merge branch 'testing'

* testing:
  packages.mk: fix file name of downloaded cmake
  doc/installation: download tarballs from halmd.org
  doc: improve style sheet, minor changes
  doc: update plotting example
  doc/usage: update page numbers for paper on H5MD
  doc: update all benchmarks for 1.0-alpha1
  doc/faq: fix emphasis in bold text
examples: add patch script to support VMD plugin

Particle trajectories can be visualised with VMD (Visual Molecular
Dynamics) [1] taking advantage of a plugin [2] (under development) for
the H5MD output files. The plugin is still a bit restrictive, the
following patch routine tries to meet these restrictions.

[1] http://www.ks.uiuc.edu/Research/vmd
[2] https://github.com/h5md/VMD-h5mdplugin
Merge branch 'testing'

* testing:
  doc: fix h5md website links
doc: update link to testing release 1.0-alpha2