refactored Markdown class to use traits

all markdown features are now in a trait so composing new markdown
flavors is as simple as adding traits to the class
started work on github markdown and markdown extra

- absy support
- traits
adjusted github markdown to use traits
final cleanup and review for default + github md
Markdown Extra
replace invalid utf8 characters

fixes #46
updated docs
Merge pull request #67 from cebe/trait-refactoring

refactored Markdown class to use traits
Update .travis.yml

allow hhvm-nightly to fail. this is currently broken somehow on travis.
added abstract methods to traits
micro optimization for indentify functions
be more GFM compatible according to lists

fixes #65
fixed endless loop bug with blockquote
removed table trait from default markdown