add new method LabeledDiGraph.trim_dead_states

add option to use it in synth
DOC: interfaces.jtlv.get_priority
DOC TRIV
DOC: Elaborate docstring of interfaces.gr1c.synthesize()
Support selection of gr1c initial condition interpretations

Fixes #84.
Bugfix: correctly referring to varibles in spec
TEST: Catch bug fixed in previous commit

0c0468e90e0359e92b1f38221b9e4828a8463d95
concerning union (__or__) of GRSpec objects.
Merge branch 'trim' of https://github.com/tulip-control/tulip-control
update copyright
fix docstring

networkx graphs are built on nested dicts
rename method LabeledDiGraph.trim_dead_states -> remove_deadends

remove is common for networkx methods

LabeledDiGraph should better be agnostic of "states" and use the term "nodes", because it is a networkx derivative. Although States is one of its attributes, it mostly wraps its functionality.
LabeledDiGraph may be split in the future into a pure-networkx subclass and a system-specific class (States and Transitions will reside in the latter only).
increase pythonism, reduce verbosity

- access nodes using iterator instead of instantiating list by calling nodes().
- check emptyness of succ[n] dict, instead of instantiating new list by calling neighbors().
add unit test for LabeledDiGraph.remove_deadends

closes #92
rename synth.synthesize option: trim_aut -> rm_deadends

- because the object returned is a transducer, not an acceptor.
- refer to the result in the docstring (w/o details about removal iterations)
- bugfix: call the renamed LabeledDiGraph.remove_deadends
TEST: Delete tulip.spec logging level setting

...so that, in the case of errors, the massive plyparser dump can
be easily ignored by --nologcapture
TEST: Catch bug introduced in 598d43e24a870f48d47de35a6e5d49e3d28c967d
BUGFIX: undefined "trim_aut" in synth.synthesize()

Bug apparently introduced in 598d43e24a870f48d47de35a6e5d49e3d28c967d
Corresponding test is given in previous commit (254a148176727b22de1544b5db897eaf6e51fda7).
DOC: Add tulip-control.org to README
Merge branch 'master' into strategy

Conflicts:
	tulip/transys/labeled_graphs.py