Cleaning up the build toolchain, round 1

 * better dependency tracking;
 * smaller Rakefile, configurable things moved to external yml files
 * split most of rake.rb mess into dedicated files.
Fixed yaml dependency name.
Fixed dependency on rust-core.
Refactored code to allow build_all.
Updated README to track new build process.
Travis will now build all apps.
No need to install pry in travis build.
Fixed typo in error message.
Rebuild on feature set change.
Merge pull request #22 from farcaller/refactor-build-toolchain

Cleaning up the build toolchain.