Fixes for OS X + Python 2.7, resolves [NPC-120] ? + addresses Marek O's concerns.

Updated build system to resolve [NPC-120], including addressing Marek O's comment/concern here:

https://issues.numenta.org/browse/NPC-120?focusedCommentId=10029&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-10029

Also improved build system to work with a Python 2.7 install on OS X, and made it "more generic" by removing hard coded paths to 2.6 (for example, in RegionImplFactory.cpp).

The requirements.txt file no longer specifies the precise version of a dependency, but instead allows all major versions to support the Semantic Versioning model:
http://semver.org/

The reasoning being that major version bumps indicate a breaking change (not backwards compatible), while all other changes should be backwards compatible. This keeps NUPIC up-to-date with the latest improvements to its dependencies, and any issues that arise will be dealt with appropriately on an as-needed basis.

Also updated OS X build with proper linker flags for 'nta_platform_python_libs' by using the `python-config --ldflags` command. This approach should be used on the other systems as well, however because I wasn't able to test on those systems I left the those platform sections alone.

Also fixed an issue where _PY27 wasn't defined and yet was needed by several files to work properly with Python 2.7.