Enables negative numbers to display to correct decimal places in table

1) More people are now using emoncms to log import/export of power, which can involve negative data as well as positive. This change enables 'negative value' data to display to correct number of decimal places.
2) Corrects number of decimal places if data is exactly 10 or 100.
Merge pull request #99 from Paul-Reed/patch-2

Enables negative numbers to display to correct decimal places in table