Throw an error when using qubits out of range for fake backends

[omarcostahamido]

this relates to #15 . I think I can create this error myself by checking the properties of the fakebackends with this:

for backend in ('FakeAlmaden', 'FakeArmonk', 'FakeAthens', 'FakeBelem', 'FakeBoeblingen', 'FakeBogota', 'FakeBrooklyn', 'FakeBurlington', 'FakeCambridge', 'FakeCambridgeAlternativeBasis', 'FakeCasablanca', 'FakeEssex', 'FakeGuadalupe', 'FakeJakarta', 'FakeJohannesburg', 'FakeLagos', 'FakeLima', 'FakeLondon', 'FakeManhattan', 'FakeManila', 'FakeMelbourne', 'FakeMontreal', 'FakeMumbai', 'FakeOurense', 'FakeParis', 'FakePoughkeepsie', 'FakeQuito', 'FakeRochester', 'FakeRome', 'FakeRueschlikon', 'FakeSantiago', 'FakeSingapore', 'FakeSydney', 'FakeTenerife', 'FakeTokyo', 'FakeToronto', 'FakeValencia', 'FakeVigo', 'FakeYorktown'):
...     backend+="()"
...     backend=eval(backend)
...     with open( (backend.dirname + "/" + backend.conf_filename), 'r') as f:
...             data = json.load(f) 
...             data['n_qubits']

with the exception of melbourne which is configured differently...

[omarcostahamido]

last 3 lines of the code above should be replaced by: FakeTenerife().configuration().n_qubits as I realized in #18