Improved support for international text

 - Implementation by @herm for GSOC 2012 (http://mapnik.org/news/2012/10/06/gsoc2012-status9/)
 - C++11 port, improvements, optimizations by @artemp
 - Testing and integration with master by @springmeyer
 - Thank you to all the support from @behdad along the way
 - Thanks for help testing @toton6868, @stephankn, @nirvn, @mfrasca, @simonsonc and many others

Refs: #2073,#2070,#2038,#2037,#1953,#1820,#1819,#1714,#1634,#1547,#1532,#1319,#1208,#1154,#1146
Merge pull request #2087 from mapnik/harfbuzz-merge

Improved support for international text