Added POT tasks

New tasks:
- `pot` - collect translatable strings and create *.pot files
- `check:pot` - check for common mistakes in translations (currently just checks for `#{foo}` substitutions)

- 0.1.10

My plan is to run `rake check:pot` at Travis builds to find the mistakes early.
we do not depend on devtools, so it would be nice if we can report useful error if it is not installed.