Added building the Play plugin along with the worksheet
Modularizes building plugins, adds Scala-Search

All plugin building optional, default is just building the worksheet (as usual).
Passing a value to the branch for any plugin assumes this branch should be built.
If the branch is empty, the user iw prompted whether

Cleaned up the logic somewhat (branch existence is now tested even for non-interactive input,
plugin repos are not cloned if the plugin is not going to be built, branch asserts fail now before cloning).

This is very much still a rough cut.
Fixes after PR review:
 boolean bugs, conditional p2 merging, ifs, bash substitution
Plugin building upgrades

Removing forcible interactive behavior for plugin building
Removing obsolete Scala 2.9.x profiles
Require confirmation for plugin building under 2.11
Updated sample scipts to reflect changes plugin usage
Documenting plugin building configuration
Have an option to bypass compatibility checking
Merge pull request #8 from huitseeker/feature/play-plugin-build

Added building the Play plugin along with the worksheet