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

NEWS

@@ -10,6 +10,7 @@ New Features in task 1.9.3
   - Now supports 'now' as a date.
   - Now defines an overdue task as being one second after the due date,
     instead of the day after the due date.
+  - Import and export of YAML 1.1, including round-trip capability.
 
   Please refer to the ChangeLog file for full details.  There are too many to
   list here.
@@ -18,6 +19,8 @@ New commands in task 1.9.3
 
   - New 'task color legend' command will show samples of all the defined colors
     and color rules from your .taskrc and theme.
+  - New 'task export.yaml' command will export YAML 1.1, which can then be
+    imported via 'task import <file>'.
 
 New configuration options in task 1.9.3