Drift and kick tasks

Particles (and cells) now carry a time for the end and for the begining of their current timesteps and not a dt anymore.

Conflicts:
	examples/test.c

Io fixes

That fixes some issues #52 that appeared on the BlueGene when reading more than 1>>32 particles in parallel or serial mode.

I have also modified the uniform box script to be much faster and lightweight. 

See merge request !48

Ensure GIT_BRANCH non-empty when not on a branch

If HEAD is detached e.g. during `git bisect`, GIT_BRANCH ends up empty, which causes an error at runtime. This change grabs the description from `git branch`, which is either the branch name or something like `(HEAD detached at <hash>)` on recent versions or `(no branch)` in older versions. Works with BSD and GNU sed, and git >= 1.0.0b (at least) although detached HEAD only introduced in 1.5.0.

See merge request !47

By default the code does not dump the task graph any more. This is dumped only hen the option '-y' is passed to the command line.

Blue gene HDF5 i/o fixes

This fixes the i/o bug #53 on the blueGene and adds a unitary test reading in a fresh hdf5 input.

See merge request !46

Get basename of branch without --short option, this requires a git > 1.7.10

Don't use --short in symbolic-ref. 

See merge request !45