gemini integration and tests for button, icon and spin

- Using `gemini 0.4.x` that support capabilities specification for every single browser
- Run `gemini` tests in Travis in parallel with `specs` and `lint` tests
- Test if FF 28, 29 and Chrome 35 only because of problems with Opera and IE browsers
  - `InternetExplorerDriver` randomly hangs (any IE version)
  - Opera 12.x can take screenshot of the beginning of the page with viewport size only — refactoring of test bundles (or hacks in tests) are needed
- Tests are ready for `button`, `icon` and `spin` blocks
- Images in `gemini/` folder are optimized with ImageOptim to save ~45% of their size
@tadatuta Should we test against one latest version only?