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Use late static binding for Parsedown::instance()

Fixes erusev/parsedown-extra#67.

This introduces PHP 5.3+ late static binding to the Singleton pattern in Parsedown.
It will return an instance of Parsedown which inherits the class which
called the `instance()` method rather than always returning instance of just `Parsedown`.

Tests are testing this feature with a test class which inherits from Parsedown.
Notice that calling `instance()` with the default arguments after an instance of
`Parsedown` was already created, it will return it even though it is from just
an instance of `Parsedown`. So this is fixing the problem just partially.

Fix check against $matches

Test on nightly releases of PHP (7.0) and HHVM

Fixes inline reference links with int 0 as reference

The link [link][0] where [0] is set at the bottom of the md file current breaks and it's truthy value is false.

Remove "tested in PHP 5.2"

It is no longer tested on PHP 5.2 in Travis CI.

See https://github.com/erusev/parsedown/pull/286

Updated readme to display travis-ci.org build test