Drafting the Pandas Section

progress so far:

• section intro
• loading data
• working with dataframes
• selecting data in a dataframe
• filtering data in a dataframe
• combining dataframes
  • working with datetime columns
• groupby and split-apply-combine
• conclusion

feel free to review as I'm writing, @ext.bauer or anyone else

This is ready for high-level review. typo fixes etc can be taken care of later, but it would be good to catch any big issues - things I've got wrong or big things missed out - before this is merged.