Cron module upgrades

- added cron_file attribute: if specified, the file with appropriate
job is created in /etc/cron.d directory. Also, you can store multiple
jobs in one file. state='absent' attribute is handled in the following
way in this case: if after the deletion of the job from the file specified
by cron_file variable the file is empty, the file is deleted, otherwise
not.
  - fixed the behaviour, when the backupfile is saved forever in /tmp
folder, even if the backup= atribute is not set (os.unlink() is called if
backup is not True).
  - added some comments to the unobvious places

test/cron_test.yml

@@ -0,0 +1,42 @@
+-
+  hosts: all
+  gather_facts: no
+  user:  root
+  vars:
+    color: brown
+  tasks:
+    - name: test 1 
+      cron: name="execution test 1" weekday="2,3" minute=0 hour=12 user="root" job="YUMINTERACTIVE=0 /usr/sbin/yum-autoupdate" cron_file=yum-autoupdate state=absent
+      tags:
+        - cron
+
+    - name: test 1-1
+      cron: name="execution test 1" weekday="2,3" minute=0 hour=12 user="root" job="YUMINTERACTIVE=0 /usr/sbin/yum-autoupdate" cron_file=yum-autoupdate state=absent
+      tags:
+        - cron
+
+    - name: test 2-1
+      cron: name="execution test 2" weekday="2,3" minute=0 hour=12 user="root" job="YUMINTERACTIVE=0 /usr/sbin/yum-autoupdate" state=absent
+      tags:
+        - cron
+
+    - name: test 2-2
+      cron: name="execution test 2" weekday="2,3" minute=0 hour=12 user="root" job="YUMINTERACTIVE=0 /usr/sbin/yum-autoupdate" state=absent
+      tags:
+        - cron
+
+    - name: test 2-3
+      cron: name="execution test 2" weekday="2,3" minute=0 hour=12 user="root" job="YUMINTERACTIVE=0 /usr/sbin/yum-autoupdate"
+      tags:
+        - cron
+
+    - name: test 3-1
+      cron: name="execution test 3" weekday="2,3" minute=0 hour=12 user="root" job="YUMINTERACTIVE=0 /usr/sbin/yum-autoupdate" cron_file=yum-autoupdate state=absent
+      tags:
+        - cron
+
+    - name: test 3-2
+      cron: name="execution test 3" weekday="2,3" minute=0 hour=12 user="root" job="YUMINTERACTIVE=0 /usr/sbin/yum-autoupdate" cron_file=yum-autoupdate
+      tags:
+        - cron
+