Adding YAML support

[zmertens]

This change is intended to add basic functionality to parse YAML files addressing issue #23 .

• I updated dawg.txt to try to explain what the YAML parser expects.
• The issue that I was having with the recombination example had nothing to do with the parser and was just a very subtle spelling error when I made the yaml test file.
• I think -DBUILD_EXTERNAL_PROJECTS:BOOL=ON needs to be on for Jenkines in order to pull down YAML-CPP and build (which should be supported by the ExternalDeps.cmake
• Did not add the part deletion functionality

yaml-callgrinds.tar.gz

[dng-jenkins]

Can one of the admins verify this patch?

[reedacartwright]

Jenkins, okay to test

[zmertens]

I updated the branch, a couple important changes from the previous branch:

• I added to the parse_test a test case to test the basic-dna.yaml file.
• I added pre-processor definitions to strip out yaml-cpp references from the CPP if the yaml-cpp library is not found
• I made some minor changes to how the YAML is parsed, namely, changing the name of some of the iterators in parse_yaml in trick_parse.h file (the it, itt, and ittt family was getting confusing), and also added a to_lower function call for verifying the file extension in parse.cpp
• I squashed the commits down