Improved handling of emph/strong. Fixes #43, #65, #78

* No more exponential parse times by the nature of emph/strong combined in PEG
* Parsing of more complex (nested) emph/strong constructs is now really close to original Markdown
Fix for emph/strong nested in other supernodes (h1,h2 etc) + support strong prefix (github style)
Updated tests to reflect new behaviour and impl of emph/strong sequences
Adds tests for emph/strong handling to scala test suite
Merge pull request #87 from Elmervc/master

Improved handling of emph/strong. Fixes #43, #65, #78