Update and refactor MacPython builds
BF: fix python to current MacPython download
BF: newer dateutil does work with Python 3
RF: update to latest MacPython 2.7 series
DOC: point out testing on 10.9 not 10.8 (anymore)
BF: fix use of GET_PIP_URL variable
BF: by default pip does not have a hyphen now
DOC: typo in comment
RF: factor out URLs, versions to globals

Update png, freetype versions
NF: set default compilers
RF: refactor CC, CXX setting
RF: continue terryfy refactoring

Continue WIP refactoring to terryfy submodule.

[ci skip]
RF: continue refactor to terryfy

Use PYTHON_EXE

[skip ci]
DOC: update README a little
BF: fix variable name for Tcl install
WIP: disable travis-artifacts for the moment

It was breaking the build
BF+RF: fix syntax for else; default to clang
BF: install bz2; fix sudo, fix py prefix

The long haul continues
BF: add flags to suppress python clang error
BF: remove ill-advised bz2 configure step

bz2 does not have a configure step.