Change directory before running job.

The queue was previously confined to running jobs in its own directory.
The Mdjob class can now take an additional "directory" keyword argument.
Specifying this will cause the script to change to this directory before
executing the job.
Updated "usage" information with a section on Mdjob.
Moved all Pyro-dependent modules from mdrunr to server.

"mdrunr.py" is now a 'backend', handling the queuing system, when to submit
jobs etc.

"server.py" is a module that wraps "mdrunr.Mdrunr" queues into Pyro
daemons to allow access from other processes.
Split mdclient into remote client and job definition sections.

"mdclient.py" contained utilities to connect to a Pyro queuing server,
and the class Mdjob which defines a job. This commit splits these
orthogonal activities. The Mdjob class is brought back to mdrunr, and
the utilities to connect to a queuing server are moved to a new
"client.py" file.
Updated "usage.md" to reflect changes in module structure.
Basic "developper's documentation".

Added 'dev_docs.md', which provides a brief overview of the internals.
Started "docs" folder.

Re-grouped all the documentation into a single folder.