remove Gemfile.lock
Add dependency nanoc
Run simple filter
Merge pull request #4 from tetor/feature/run_filter

Run simple filter
Check environment is ready or not

Whether 'dot' command istalled.
To be green if the Graphviz is not installed
Check whether preparation of running filter is ready

And rename some methods according to Rubocop Style.
Merge branch 'feature/check_to_be_prepared' into develop
Add Rubocop development dependency
Chanage coding style at Rubocop suggestion
Fix test
Merge pull request #7 from tetor/feature/rubocop

Rubocop setup
Empty Commit: filter graphviz
dot -> png filter
Merge branch 'feature/filter-graphviz' into develop
wip:
Fix ready? method test error.
write how to use docs
Merge branch 'feature/document' into develop
Change run method returning value