Slidify rCharts integration and External Libraries about rcharts

[ramnathv]

On a related note, using options to control paths is dangerous, since the setting is invisible to the user and can cause all types of errors. So a better fix is to add a field to the reference class that stores LIB_URL. So for libraries in rCharts, it will mean LIB_URL = system.file(package = 'rCharts')

[ramnathv]

The trouble is that sometimes, communication is required with functions outside of the reference class. For example showOutput needs to know the path to the assets so that addResourcePath is correctly initialized. In such cases, options seem to be a better way of setting configuration.

This needs deeper thought, since I want to keep it as simple as possible. Ideally, the user will only have to set one thing: path to their library.

[timelyportfolio]

also duplicated in #163