* examples/validators.py: avoid redefining 'sid' in sample validator definition
* README.rst: expand explanation of validator functions
* schoolutils/grading/calculator_helpers.py: add filter_nan option to several functions
  - it seems that None and NaN have slightly weird comparison behavior,
    which throws off min, max, and histogram calculations
  - filter_nan option now allows removing these values in relevant functions
* schoolutils/reporting/reports.py: make histograms work with numeric grades
  - filter_nan for min, max, freqs_for_numbers
  - bugfix: grade type is 'percentage', not 'percent'
  - extend width of bin template in histograms to accommodate percentage bins
* CHANGES.txt: summary of changes for 0.1.7