improve parsing of list item and code block by measuring line indentation
dense list items that follow sparse ones should not be rendered as sparse ones
add contributing guidelines
improve consistency of list item
improve contributing guidelines
outdented is shorter and probably more accurate
simplify em/strong routine

retry to search em/strong markers to fix nested em/strong correctly
simplify testing and improved output
Merge pull request #84 from wkpark/fix-em-strong

simplify em/strong routine
improve contributing guidelines
Merge pull request #85 from cebe/simplify-tests

simplify testing and improved output
resolve #89
resolve #88, resolve #81
reference labels should be case insensitive
resolve #82
fix reference
simplify reference
improve tests
resolve #92
Run the testsuite on PHP 5.6