parse document directly instead of chunking into section lines

- remove the section reader and instead parse the sections off main reader
- add parse method that handles parsing logic previously in document
- parse downwards from sections rather than through the next_block method
- add mode to next_block method where it only looks for blocks
- make section title parsing more efficient
- rearrange some of the methods in Lexer
- rename heading_* symbols to section_*
- add tests for block level
- TomDoc
- some source code tidying
add test for block title carrying over from section to first block
fix source of infinite loop
support square & round brackets in front of link, round brackets around a link
set asciidoctor attribute
prevent empty dd element for item w/ multiple terms
make sure nested labeled lists are found when indented

- nested labeled lists may occur in indented paragraph text
- but not if literal paragraph is attached by list continuation
prevent literal & indented content from being too greedy

- fix literal content in a list item from consuming following list item
- fix indented content in labeled list from consuming following content
- more list tests
- add convenience methods for checking if blocks, text or title exists
change keep_if to delete_if for ruby 1.8.7
additional list test for title on following block
Merge pull request #71 from mojavelinux/list-fixes

Fix additional bugs uncovered by rendering user guide