removed allowed failure for PHP 7.1 in Travis build matrix

removed unnecessary FIXME comment

added missing UTF-8 qualifiers for multibyte functions

fixed issue with syntax tags in the shortcode content: they caused Re…

…gularParser to return empty content as if shortcode was self-closed

text length must be recalculated after applying each shortcode replac…

…ement because of the convoluted replacement logic due to the issues with multibyte strings

That's another level in the mastery of maintaining sanity while still…

… supporting PHP 5.3. Way to go, man, way to go...

RegexParser no longer matches shortcodes with name containing undersc…

…ores

added another test case for invalid shortcode

updated changelog for release 0.5.1

updated README for version 0.5.0