Fix a possible nil exception in plural_keys.rb
New tasks

i18n-tasks health displays missing, unused, and some more information
i18n-tasks tree-* manipulate trees
i18n-tasks data-* manipulate store
Refactor commands DSL
doc
get rid of "null" node
Merge branch 'pr-79' into data-commands
refactor PR #79
refs #78 allow punctuation in scanned key names

bump version to 0.7.0