Set default-directory to project top directory

Because 'motion-bundler-p' should be called at project top directory.
Fix setting to unexpected value

In asynchronous case, motion-rake-task-list-cache was set to
return value of motion-get-rake-tasks-async. It was not expected value,
because it is return value of set-process-sentinel.
Its value should be nil until asynchronous task is completed.
Merge pull request #16 from syohex/fix-bug

Fix some bugs