Vignette develop

[itismeghasyam]

• Resolves #179 (I didn't write the tests because commenting out that code totally eliminates that problem)

• 144 - Sensors and Modules vignettes are now updated to reflect new architecture of the package. Also Phil can you please include examples for using funs and models in these vignettes ?

• I also made changes to heartrate_data. It now has only columns t, red, green and blue and a much better illustrative record. Before we had some record that was not as illustrative and also we had extra columns of brightness, hue and saturation.

• 185 I reduced the number of examples in many feature extraction functions, and they also run on a subset of the sensor data to make the processing faster. The travis build now takes ~15min instead of ~18min

[philerooski]

BTW, you don't need to close a PR if you need to make changes. Just push new commits to the reference branch (in this case, itismeghasyam:vignette-develop) and they will be reflected in the PR.