Dependency on scala-xml removed. Scala.js integration.

Hello, here is the pull request for the changes we´ve been discussing last days. To summarize:

- Removed the use of symbols to define tags. Substituted by String's.
- Removed the dependency on ```scala-xml``` and created a proper ```.toString``` method.
- Added ```scalatags-js``` alternative SBT project for Scala.js integration.

And.. happy new year!