Added get_data_range ajax function.

Returns the maximum and minimum values to be found on a map.
Map now shows full range of values.

The computation map will now automatically adjust the colour range to
represent the entire gamut of values in a data file.
Add legend to map

The map will now display a legend in the upper-right hand corner.
Merge pull request #14 from Andrew-Dunn/legend

Added legend to map.