New backdrop - fixes

Resolves the issue described in #38

Prepare for semantic versioning & npm package releases

With thanks to @tszulc for doing this.

• Moves change logs from README to CHANGELOG, using Keep a Changelog format
• Moves documentation from bootstrap-tourist.js to README
• Adds a package.json
• Adds conventional-changelog/standard-version package in preparation for semantic versioning

New backdrop

Major revision to backdrop approach

new method of backdrop and highlight added, with major effort and code from @ibastevan.

Full changelog can be found in the top of bootstrap-tourist.js

v0.3.0 represents a significant change to Bootstrap Tourist.

Tourist is still completely drop-in replacement compatible with original Tour,
however @ibastevan and @IGreatlyDislikeJavascript have made significant changes
to how the backdrop works. You can read the conversation here:
#24

In short, previous versions of Tourist (and original Tour) used 4 divs to bracket
the tour step elements.

From v0.3.0, Tourist now uses overlays to produce the same effect. This solves
many problems and enables many new features, including customizable transitions
between tour steps.

There may be residual bugs from this change so please report them in
the repo!

Rollup for 0.1.x series with additional fixes

Rollup for the 0.1.x series of Tourist, with additional final fixes to begin the 0.2.x series.

BS3.4.x and 4.x compatibility, first fully working beta

This checkpoint covers major fixes from 0.0 -> 0.10. This release is considered the first fully working beta of Tourist, before next round of feature implementation.

Key changes for implementers

• Provides full Bootstrap 3 and 4 compatibility
• Fixes for Bootstrap sanitizer stripping issues
• Significant changes to management of popovers, triggers
• Significant changes to integration with BS4 popper.js to provide properly "center and stick" orphan popovers
• Changes to popover templates, object properties etc for BS4 compatibility
• Changes to the way background overlay is implemented, paving the way for better overlay in future versions

Full details in the repo and at the top of the .js. Please note the readme in the download zip/tar is NOT the latest one, due to my mistake with commits. Please always refer to the notes in at the top of the js.