Fix a bug in RechenbergOptimizer

I also removed an old and useless example.
Add new parameter type `param.multi`

This new parameter type can be used to add multiple parameters at the
same time:

@param.multi(param.bool, ["a", "b"])
def f(a, b):
	...

Is equal to:

@param.bool("a")
@param.bool("b")
def f(a, b):
	...
Fix issue #15
Merge branch 'develop' into renke-multi-param

I also simplified the one_{min,max} objective functions using the new
parameter type multi.

Conflicts:
	examples/one_min_gridsearch.py
	metaopt/core/paramspec/util/exception.py
	metaopt/core/paramspec/util/param.py
	metaopt/tests/unit/core/param/param.py
Merge pull request #16 from cigroup-ol/renke-multi-param

New parameter type `multi`