New version 0.2.0

New features:

• Separated alda-rb.rb into several files.
• REPL now supports map, score, and new.
• Added a lot of docs. Can be seen here.
• Added Alda::Event#detach_from_parent.
• Fixed the bug that dot accessor of Alda::Part does not return the container (or the part itself).
• Added Alda::LispIdentifier.
• Fixed Alda::EventList#import now returns nil.
• Added some unit tests.
• Fixed the bug that creating an Alda::GetVariable occasionally crashes.
• Fixed bug that inline lisp is mistakenly interpreted as set-variable.