Added chart specific handling of data labels.

Issue #170
Added separator option to chart data labels.
Added legend key option to chart data labels.
Added updated docs for chart data labels.
Added number formatting for data labels.

Issue #130
Added font handling to chart data labels.
Fix for worksheet name quoting in worksheets and charts.

Issue #167
Prep for release 0.6.1
Minor doc fix.
Minor doc fix.
Added chart axis line and fill properties.

Issue #88.
Prep for release 0.6.2