Add 'polite' YAML syntax validation and hinting

[tbielawa]

Closes #798

[tbielawa]

Example output:

<tbielawa>@(deepfryer)[~/Projects/ansible] 18:25:19  (yamlvalidation) 
$ ansible-playbook -v ~/testyamlfix.yaml                                   
ERROR: Syntax Error while loading YAML script, /home/tbielawa/testyamlfix.yaml
Note: The error may actually appear before this position: line 4, column 9

  tasks
  - name: tell us what distribution this is
        ^

Given the invalid playbook:

--
- hosts phx2
  tasks
  - name: tell us what distribution this is
    action: command cat /etc/redhat-release

[mpdehaan]

sharp!

[mpdehaan]

merged, thanks!