add an API to create belief structures.

This will ease the parsing of new types of files and will add the
possibility to create structure on the fly later.

Currently the function do not really check for consistency in the
belief structure, meaning that you can add a sensor model which
contains beliefs about another frame of discernment for instance.
put sources in "core" folder to separate core library from other components later.
Added function to copy a BFS_SensorBeliefs.

It is useful if you have a common configuration for several sensors
as you can create it once and then copy it with a different name.

Also made a fix in BeliefFromSensors.c
Added an xml parser for the configuration.

The xml format will allow to have one file containing everything needed
for a model. Currently there is no verification via xml schema or DTD.

The new parser is compiled as another library depending on THEGAME in
order to avoid adding dependencies to the core part of the library.
Someone who wants to use xml with THEGAME will have to link to
THEGAME-xml.
Merge pull request #6 from TACOMA-team/xmlmodel

add XML model