Reimplementation of the chart node using angular-chart.js

[DanCunnington]

Fix for #38

Summary

• Reimplemented the chart node to use angular-chart.js instead of angular-nvd3-directives. This is due to a memory issue in angular-nvd3-directives and also discontinued support.

Known Issues

• Charts render incorrectly when data points fall outside a specified range. Issue raised against Chart.js and failing test case created outside of Node-RED.
• The last x Axis label can sometimes overlap. Outstanding PR on Chart.js to fix this issue.
• Colours are manually specified - this may change when the theme chooser is implemented.

Note. This is a revised version of #89 to account for limiting the number of points as discussed in #38. This PR is ready for review.