Detect iconv for libxml2 just like the way libxml2 does.

CPPFLAGS/LDFLAGS/LIBS are now built from --with-iconv-*/--with-opt-*
flags given to extconf.rb and passed to libxml2's configure script.
Detect and help user fix a missing /usr/include/iconv.h on OS X.

This should address #1111.
Fix a typo.
Merge remote-tracking branch 'origin/libxml2--with-iconv'
Use ruby 2.1.3.
Try OS X on Travis-CI.