Databaker: load xlsx files + cell properties

[CharlesRendle]

At the moment databaker cannot read xlsx files and store cell properties because of an update to one of its dependancies - xlrd.

• [x] UML breakdown to visualise the relationship between xlrd, messytables and databaker
• [x] Sketch explicit BDD tests to determine file type (xlsx focused) and cell property functionality in databaker
• [x] Complete code to achieve xlsx functionality - reading files + cell properties without the xlrd dependancy.
  • [ ] Catch exception raised by dodgy xlsx files - convert into xls and continue with that table loader
  • [ ] Change fixture file in main test suite so that duplicate testing for xlsx doesn't get caught by above
  • [ ] Implement last cell property functions for xlsx to match xls (any_borders, all_borders, richtext, is_date)