Add LMU example

[tbekolay]

This is literally a copy of the Nengo core example, with some additional text to note that this runs on Loihi, and with some changes to parameters to make it run in a reasonable amount of time with reasonable performance. I also bumped up the learning rate a bit and upped pes_error_scale to be able to get decent performance running for only 30 simulated seconds. It still might take a bit too long on the emulator for TravisCI, but we'll see. I haven't tried running it on the board yet, but I'll do that next.

For reference, here are the plots you get at the end with Nengo core and below are the plots I got running this on the emulator:

I'll post the plots from the chip when I have them.

[tbekolay]

Here are the plots from the chip; worked first time without any changes, which is nice! Performance is about the same as emulator, but faster, which is also nice.

[drasmuss]

Updated plots