small cleanup

- created and applied two utility functions (cleaner code, more minify-friendly):
  - `isFunction`
  - `isDefined`
- simplified some `if` checks
Merge branch 'patch-2' of github.com:stryju/SVGInjector into stryju-patch-2

Conflicts:
	svg-injector.js
Merge branch 'stryju-patch-2'

 to explain why this merge is necessary,