add https://github.com/mapbox/mapnik-test-data to tests - /cc @GretaCB
travis: debug mapnik-test-data
fix angle calculation

Conflicts:
	src/text/vertex_cache.cpp
update visual tests after #2399
travis: more debugging of mapnik-test-data
Allow user to specify a python executable.¬
¬
Fall back on `python`. Unlike the solution in PR #2395 this¬
puts all the responsibility for selecting the right python in the¬
case that "python" is Python 3 on the user. I've followed the lead¬
of the Python documentation Makefile. See

http://hg.python.org/cpython/file/340d48347295/Doc/README.txt
http://hg.python.org/cpython/file/340d48347295/Doc/Makefile

Intent is that if your `python` is Python 2 everything works as
before and that is your `python` is Python 3 but you have a
`python2` you can specify that instead.¬

Conflicts:
	run_tests
remove copy converting ctor and fix converting move constuctor to use 'universal reference' and perfect forwarding
( re : http://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers)
re-implement mapnik::value deriving from value_base and update across core
fix mapnik::value conversions in topojson plugin
Merge branch 'master' of github.com:mapnik/mapnik into 3x-msvs