Add support for DocBlock template markers

DocBlocks may start with #@+ and #@- to indicate that they are (the start) of a DocBlock
template or the end of a template.

In this commit I have changed the way a DocBlock is parsed to interpret this information
and added tests to show for it. In addition I have added more comments to the Regular
Expression responsible for splitting a DocBlock to show the business rules more clearly.

This is the first step in implementing https://github.com/phpDocumentor/phpDocumentor2/issues/42.