[make] check2.7 -> check2
[r_value] Drop support for covariance

The R value only needs variances, for D large, the difference between
covariance matrix ~D^2 and variance ~D is significant.

o set default critical R value to 2
o remove `multivariate_r`
o update doc strings
o update example `r_value`
[linalg] Add chol_inv_det

Use direct access to lapack via scipy to compute the inverse and
log(det) via the Cholesky decomposition. This is both faster and
more robust than np.inv and np.det, which work on non-symmetric
matrices, too.
[density, mix_adapt] Use chol_inv_det in LocalGauss
[variational] Use chol_inv_det

Some more members to compute inv and det only once. Det now computed on
the log scale and much more robust against overflows

o output 15 digits on bound
o change interface of Wishart*
[variational] bug fix string comparison: 'is' -> '=='
[mixture.pyx] start docstrings with newline such that cython correctly includes the call signature
[doc] correct reference [Wra+09] -> [Kil+09]; [Wra+09] explains PMC, not adaptive MCMC
[markov_chain] remove todo comment
[ImportanceSampler] introduce "clear()" function like for "MarkovChain"
[markov chain] optionally save target evaluations

this commit solves part of issue #35
[importance sampling] optionally save target evaluations

this commit solves part of issue #35
[importance sampling] optionally save target evaluations

this commit solves part of issue #35
[doc] section "How to use this documentation": pypmc --> ``pypmc``
[Makefile] run all tests with "mpirun -n 2"

if a new MPI-paralellized function is added, it is always tested;
in particular, we are not required to explicitly add it to the Makefile
[markov_chain, importance sampling] no op for run(0)

raise an error if sampler shall produce a negative number of samples
do nothing if sampler shall run for zero steps
[plot_mixture] add argument ``visualize_weights``

If ``visualize_weights=True`` coloring is based on component weights
[example] visualize the component weights in final plot of "MCMC + VB"
[doc] "calculates ..." -> "calculate ..."
[tools] add parallel_function