Allow any value range on logarithmic scale

[bear454]

The prior implementation required values > 0 for proper operation; by offsetting to values starting at 1, any range can be used with the logarithmic scale.

Resolves #835

JSFiddle example: https://jsfiddle.net/j1LLdrhq/5/

Pull Requests

Please accompany all pull requests with the following (where appropriate):

• [x] unit tests (we use Jasmine 2.x.x)
• [x] JSFiddle (or an equivalent such as CodePen, Plunker, etc) or screenshot/GIF with new feature or bug-fix
• [x] Link to original Github issue (if this is a bug-fix)
• [x] documentation updates to README file
• [x] Passes CI-server checks (runs automated tests, JS source-code linting, etc..). To run these on your local machine, type grunt test in your Terminal within the bootstrap-slider repository directory

[rovolution]

@bear454 merged and published to v10.1.0. thanks for your contribution!