add dplyr tutorial to Getting and Cleaning Data

My pull request is for an additional link to the Getting and Cleaning Data page. I created a 40-minute video tutorial (and a related R Markdown document) that teaches the fundamentals of the dplyr package in R.

As for the questions:

1. Does my contribution teach? Yes, I actually taught this exact material to a real-life data science class.
2. Does the content of my contribution clearly address topics in the Data Science Specialization? Yes, dplyr is directly relevant for week 3 of Getting and Cleaning Data.
3. Could my contribution be seamlessly integrated into the canonical course materials? Yes, you could teach almost all of week 3 using dplyr instead of using plyr and base R. And in fact, I bet the instructor would have used dplyr for week 3 had it existed at the time the videos were recorded! (dplyr was launched in January 2014).

Thanks! I'm happy to answer any questions and/or revise the text if desired.