Hackathon squash

- This cl is mostly concerned with extending ToPOD and ParseArg
methods to support precise serialization/deserialization.  ToPOD
now takes an optional "precise" argument (default true) -- when
this is set to true, FT and RT leaves of serialization tree are
serialized to precise string representations dependent on the
kernel in use.  When the precise argument is false, FT and RT
leaves are serialized to doubles, and precision may be lost.
ParseArg methods now understand either format.

- A wrapper has been added for Aff_transformation_2.

- A wrapper has been added for the global collinear function.

- A coordinate accesser has been added to the Polygon_2 wrapper.

- tavis.yml has been modified so travis will matrix build with both
the epick and epeck kernels for acceptance testing.

- binding.gyp has ben modified to explicitly build with libc++ under
OSX, in order to address some dynamic linkage issues surrounding
use of CGAL stream operators in "pretty" mode.

- A first pass has been made at making commentary and header
formatting more consistent.