Add continuous integration testing using Travis CI

I set up test suite using free service [Travis CI](http://docs.travis-ci.com/user/getting-started/)

What tests currently cover:
* 6 versions of PHP *times* 2 kind of databases = 12 testing environments
* linting of all CSS files [and fixing of found bugs: https://github.com/kuc/Tiny-Tiny-RSS/commit/0b92318bd807e5cc07dc2d4ba2e7c074648bdc76 ]
* syntax check for all Javascript files
* validation of `*.xsl` files
* syntax check of all PHP files
* Tiny Tiny RSS instalation process, using GUI
* test if all migrations from `4` to `LATEST_MIGRATION` run without errors
* headless browser functional testing: login, logout, updating feeds; uploading page screenshot on Imgur after failure for easier debugging - [example](https://i.imgur.com/Yv7MVpq.png)

Preview: https://travis-ci.org/kuc/Tiny-Tiny-RSS

-------

PS: enabling option
>Build only if `.travis.yml` is present

on page `https://travis-ci.org/GITHUB_NICK/Tiny-Tiny-RSS/settings` is required for proper functioning of Travis