Revealing `availableWidths` bug through tests.
Proper initialisation of Imager instance `availableWidths` option.

- removed `isArray` function
- bootstrapping instance variables
- array/object paths
Merge pull request #66 from oncletom/fix-66

[READY] README demo for defining availableWidths as a function does not work
Merge remote-tracking branch 'upstream/browsers-compatibility'