'exclude' configuration option seems to be mandatory

[spier]

I created the _epub.yml exactly as outlined in your README:

epub:
  name: "My Jekyll Blog"
  lang: en
  identifier: my_blog.example.com-123456

When running jekyll_epub I got this error:

** Configuration from [...]/_epub.yml
[...]/.rvm/gems/ruby-1.8.7-p352/gems/jekyll-epub-0.0.3/bin/../lib/jekyll/epub.rb:111:in `package_epub': undefined method `include?' for nil:NilClass (NoMethodError)
    from [...]/.rvm/gems/ruby-1.8.7-p352/gems/jekyll-epub-0.0.3/bin/../lib/jekyll/epub.rb:106:in `each'
    from [...]/.rvm/gems/ruby-1.8.7-p352/gems/jekyll-epub-0.0.3/bin/../lib/jekyll/epub.rb:106:in `package_epub'
    from [...]/.rvm/gems/ruby-1.8.7-p352/gems/jekyll-epub-0.0.3/bin/../lib/jekyll/epub.rb:76:in `epub'
    from [...]/.rvm/gems/ruby-1.8.7-p352/gems/jekyll-epub-0.0.3/bin/../lib/jekyll/epub.rb:235:in `create'
    from [...]/.rvm/gems/ruby-1.8.7-p352/gems/jekyll-epub-0.0.3/bin/jekyll_epub:103
    from [...]/.rvm/gems/ruby-1.8.7-p352/bin/jekyll_epub:19:in `load'
    from [...]/.rvm/gems/ruby-1.8.7-p352/bin/jekyll_epub:19

Line 111 of epub.rb assumes that the exclude configuration option will always exist.

When adding an empty exclude option to _epub.yml the epub generation worked just fine.

# A list of directories and files to exclude from the conversion
exclude: []