Trending Item Recommendation

This pull request introduces time-series trending analysis as an item recommendation engine. As opposed to the one currently in the trending_algo branch, this one does a series of regressions and has an optional filters to smooth the data. The minimum number of parameters are exposed to the end user because we assume they don't have extensive knowledge about time series analysis (although we can change that later).
As it sits right now, the Trending.scala test files have yet to be written, but should be done soon by Haoyuan.