Merge branch 'exp' into develop
Fixed a Notice caused by relying on an undeclared variable
Update readme.md

Changed the build status image src and link to the new travis-ci url.
Added documentation to the DefaultTaskBuilder
Update composer dependencies: added react/promise
refactorings to Amaka, added promises to the runner.
Start refactoring of amaka
Replaced the RecursiveRunner with the AbstractRunner
Removed a check in the AbstractRunner (already invoked tasks don't get invoked anymore)
Changed the File task and factory method.
Fixed wrong merge of exp on master (now on develop).
Removed the setBuildfilePath method from the Amaka class
Minor changes to the Amaka class (regarding the coupling with Context)
Moved all the code of the CycleDetector to the Dag class, which still couples Amaka with Facebook libphutil
Renamed namespaces and directores
Replaced all occurrences of Buildfile with AmakaScript (in the code)
Deleted unnecessary documentation
Added the PluginBroker