New parser architecture.

Closes #114. This PR introduced new parser architecture on our way to proposed [Parser Components](https://github.com/apiaryio/snowcrash/wiki/Parser-Components). 

This PR separates Markdown-AST producing into a standalone library – [Markdown Parser](https://github.com/apiaryio/markdown-parser).

## Concept 
Snow Crash parser uses the Markdown Parser to parse the input Markdown buffer and thus create Markdown AST. This tree is then processed by the Snow Crash. The processing of the Markdown AST tree is done traversing the tree "top-down" splitting the concerns into [API Blueprint section](https://github.com/apiaryio/api-blueprint/blob/master/API%20Blueprint%20Specification.md#def-blueprint-section)-specific processors (`SectionProcessor`). 

All of the section processors have uniform interface invoked by the `SectionParser::parse()` routine. This routine uses processor's specific "adapters" (e.g. `HeaderSectionAdapter` or `ListSectionAdapter`) to iterate over different Markdown structures. The parser in conjunction with processors produces the out blueprint AST as defined in [Blueprint.h](src/Blueprint.h).  

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Related links:

- [GitHub: Refactor Regression Issues Milestone](https://github.com/apiaryio/snowcrash/milestones/Refactor%20Regression%20Issues)
- [GitHub: Refactor Progress](https://github.com/apiaryio/snowcrash/wiki/Refactor-Progress)
- [Trello: Product – Parser Architecture Refactor](https://trello.com/c/Yd4S7lcq/67-parser-architecture-refactor)
- [Trello: Project – Snow Crash Refactor](https://trello.com/c/RcdTX9Dy/1657-snow-crash-refactor)
- [Trello: Regression Tests](https://trello.com/c/ahJn7lxB/1826-run-regression-tests-on-production)
- [Gist: Parser Architecture Concept](https://gist.github.com/zdne/55b7a38f52ca642e123b)

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Note: While this PR has a lot of changes a massive number of new slocs was introduced with the performance testing fixtures (e.g. they are not code-related, there will be actually much less code in the codebase after this PR).
The exact type of the payload section (`RequestSectionType` vs `RequestBodySectionType`) is indicated by the presence of a `+ Body` (or `+ Schema`). Therefore you need to iterate over all nested list items and check whether they are recognized or not.