Feature #244, #272, #274, #275

- Implemented export.yaml, import (yaml).
- Updated man page.
- 'export' is now an alias to 'export.yaml'.
- Added missing 'tags' attribute as an internal Att.
- Improved recognition of YAML.
- Added unit tests for export.yaml, import (yaml).
- Added missing unlink from dependencies.t

ChangeLog

@@ -8,6 +8,8 @@
    as relative to the current date/time.
  + Added feature #189, that records the start and stop times
    as an annotation for a task.
+ + Added features #244, #272, #274 and #275, which support import and
+   export of YAML 1.1.  YAML is now the default export format for task.
  + Added feature #391, now the 'task color legend' command will show
    samples of all the defined colors and color rules from your .taskrc
    and theme.