Update the API (internal & external)

[t0xicCode]

Currently, both public methods and private methods are exposed and follow the same API guidelines.

In a commitment to semver, I propose for the 2.0 release, to update the API as follow:

• All public methods are assigned to an instance variable in the Selectpicker object
• All private methods follow the convention of starting with an underscore
• Implement the noConflict API

[t0xicCode]

I've reconsidered the position. At this point, we'll keep the API as is. If a method is documented on the documentation pages, the API is considered stable. All undocumented methods are subject to change within minor releases.