Bump up version to 0.2.8.
Introduce build command history length.

Please set "-Dcom.asakusafw.shafu.ui.history.limit=<size>" to
your eclipse launching configuration after "-vmargs".
Change key bindings for MacOSX.

Using MacOSX, now 'Build with Task Names' command is
binding to 'Ctrl+Option+B' (formerly it was 'Option+Command+B').