Add createCustomWave function to Sequence object, add instructions to RE...

...ADME, and add test for it

Made sure all code conforms to style guide specified via Grunt task as well, and added a test for it

I also took out the line where I initially set waveType to "square" to save a few bytes - I think calling it out in the README is clear enough. The one other thing I did was to make the API easier by making "imaginary" optional - if it isn't specified, I just copy the real array and use it for both (this is fairly common usage across examples on the web)