Make :xpath and :css equal

This pull requests brings the following on the table:

1. `:css` and `:index` selectors can be combined (fixes #241)
2. `:css` works on all container methods (fixes #124)
3. `:css` and `:xpath` selectors cannot be used together
4. Removes dead code in xpath builder where index is appended. I still think this should be restored both for XPath and CSS builders, but it's a separate thing to discuss.

Changes for watirspec are in my [fork](https://github.com/p0deje/watirspec/tree/css-emancipation). Please take a look too.

@jarib @jarmo There might be some edge cases or anything that simply didn't come to my mind, so please review the changes. I've also tested this manually a bit. I'd appreciate any code improvements too.