[tools] Add logsumexp implementation to support Ubuntu 12.04
[mixture] Use own logsumexp, evaluate can return individual components' values
[plot] Default parameter indices in plot_mixture
[variational] First cython version
[variational] Complete cython port

o variational remains a python class
o fix VBmerge
o remove tests of log_rho: now has different normalization to speed up bound calculation
o remove unnecessary burden in test_large_prune()
include cython in scripts and setup

o [setup.py]              added "cython" to "install_requires"
o [scripts/tidyup.sh]     delete library objects and cython associated tmp
o [scripts/build_inplace] run the build commands for python 2 and 3
[examples/variational.py] added more plots and comments
[mix_adapt/pmc] bugfix, cleanup and added Rao-Blackwell sensitive test case

o removed unneccessary definitions from pmc_test (i.e. target, sampler)
o added samples from overlapping components such that the Rao-Blackwell
  option matters
o calculate_rho_.. and application of ``mincount`` criterion separated
o during the updates perform check "weights is None" only once