Remove repeated functions in MLS.R and ITS.R

GetPercentageDiff and GetDate functions removed from MLS.R and ITS.R
Add IRC functions to get data about trends

Add functions to get the trends of messages sent and authors active
during last 7, 30 and 365 days.
Merge pull request #30 from VizGrimoire/irc_trends_28

Add trends for IRC messages and authors for 7, 30 and 365 days