Edits for cli pull request

- move imports to bin/asciidoctor
- set built-in attributes for doc info (set in load API down the road)
- output to stdout if out file is '-'
- calculate output file based on input file if not specified (to comply w/ AsciiDoc)
- change attribute keys to strings (as Asciidoctor expects)
- change doctype options from symbols to strings
- make doctype apply to all backends
- add options for backends
- add option to enable compact output
- add option to set safe mode levels
- make suppress header_footer work
- add option to set eRuby implementation
- automatically set value to 1 for attribute w/o value
- add files to gemspec

- disabled the invoker test because it wasn't clear what it was testing