Added support for 'hidden' config properties to hide DataSet by default

[simonc92]

I wanted to hide a specific dataset inside my django-jchart but hidden wasn't supported.

ValueError at / Use of illegal keyword arguments for DataSet: {'hidden'}

This tiny code enhancement enables this feature. After loading the page the dataset I wanted to be hidden was hidden and you have to click to show the chart.

Code exmple:

from jchart import Chart

class LineChart(Chart):
    chart_type = 'line'

    def get_datasets(self, **kwargs):
        return [{
            'label': "My Dataset",
            'data': [69, 30, 45, 60, 55],
            'hidden': True
        }]

[coveralls]

Coverage remained the same at 95.354% when pulling f9b2ddc on simon-czychy:master into 2e224f0 on matthisk:master.