move deprecated methods to the bottom of the class
simplify test class
make parser class more extensible
improve readme
remove CONTRIBUTING.md
replace tabs with spaces
texts should not share reference definitions
nested calls to lines should not reset reference definitions
fenced code blocks should respect indnetation
resolve #154
tests should use autoloader
"complete" calls should be more consistent
Run composer install in Travis CI before tests

Tests are using the Composer autoloader since: https://github.com/erusev/parsedown/commit/cd1c030362351d6006f0c2cbe52b7fbd65ca9ef8

Because of that Composer should have actually dumped the autoloader in the `vendor/` folder,
before the tests are ran.
improve code consistency
travis should be able to run tests in 5.2
remove composer dependency
definitions are not blocks
remove unnecessary nesting
improve naming consistency
improve readability