- #290
- remove workaround for aspnet/dnx#2566; not needed if building with CLR or Mono
- install Core CLR DNX in `build.sh`
  - must be applied in all repos using Travis before this PR goes in
- add `Dnu()` and `Dnx()` macros
  - remove `K()` macro from `_k-test.shade`; use `Dnx()` instead
- incorporate per-repo changes not yet applied to Universe templates
  - use newer `NuGet.exe`
  - Universe itself does not need the Korebuild package or `dnvm`

Changes partially revert 618a73b3 e.g. always start build with CLR or Mono DNX

nits:
- reduced DNX switches and `dnvm` calls in `build.cmd`
- base NuGet skip decision on _both_ KoreBuild and Sake's installation status
- make `Quiet` variable work more reliably; small changes often led to compilation problems
  - requires a small change in DNX repo at the same time
- remove some trailing whitespace and tabs
- add a bit more description to `_k.shade`