Change the structure to a npm package style

[Pliavi]

None of the original code of the library was changed, but it opens to future modifications and improvements as it can be used as and with npm packages.

Changes

• Source files moved to src/ folder.
• The dist/ folder contains only the files the user need to implement the library and the example, normal and minified versions are packed in the jbility folder.
• The assets/ folder is just for the repository, non related to the library.
• Build is done with gulp:
  • build: yarn/npm build
  • watch: yarn/npm watch
• Added contrast example to readme.md
• It's ready to be published in npm

Fork: https://github.com/Pliavi/jbility

Sorry for a "breaking change" without notice you first, hope you like it! :smile:

[urielcaire]

Thank you for your great contribution! @Pliavi