Add Gray code to modems : close both veeresht/CommPy#100 and veeresht/CommPy#60.

[BastienTr]

Implementation based on the code from @edsonportosilva in #100. I refactored the code and simplified it a bit.

The Gray code is the new default for both QAM and PSK modems. I have no time to add tests based on performances but modem.plot_constellation show a proper Gray code.

I'll merge this PR as soon as @veeresht, @kirlf and/or @edsonportosilva validate my code.

Close both #100 and #60.

[edsonportosilva]

@BastienTr, here is a notebook to run a quick BER vs EbN0 test to see if everything is ok with the Gray mapping, i.e. if the Monte Carlo simulation agrees with the theory curves.

CommpyTest_BERvsEbN0.zip

[BastienTr]

ToDo list for this PR

• Fix the Travis issues with custom modulations.
• Fix Wifi80211 Travis errors. The observed performance is better than the one in the test since the Gray coding addition. The line is commented "From previous tests". @eSoares did you run the previous tests with Gray coding?
• Implement a complete performance test for modulation based on @edsonportosilva's code. I'm on it, thanks for the input!

[coveralls]

Coverage decreased (-3.02%) to 79.777% when pulling c373d79 on BastienTr:master into 46512b9 on veeresht:master.

[BastienTr]

I removed the test_wifi80211 file since the references values seems to come from previous CommPy test. I feel like it is too risky to check a code based on result of the same code and we have an example here.
Can someone double check the PR so that I can merge it?