Update GH page for Scalate 1.7.0

- Update headers
- Scala 2.10 & 2.11 instead of 2.9 & 2.10
- Add sbt installation on the download page
- Removed distribution releases on the download page because 1.7.0 is not provided

Preview: http://seratch.net/scalate/ (seratch.github.io/scalate/)