added rna-seq data analysis lesson

Added two lessons on RNA-seq data analysis. Part I (QC, alignment, & counting) assumes basic familiarity with the Unix shell. Part II (differential expression analysis) assumes basic familiarity with R. I ran this as a [one-day workshop](http://bioconnector.github.io/workshops/lessons/rnaseq-1day/) where in the morning we did an intro to Unix followed by the first lesson here, and in the afternoon we did an introduction to R followed by the second lesson here. More details in the README here, and more about the previous workshop on [this SWC blog post](http://software-carpentry.org/blog/2014/11/virginia-workshop.html). Some lesson material was contributed by @rbeagrie on an outstanding PR to SWC based on a recent Mozilla sprint.