Use $CC or cc to compile libreadline on Darwin

Force the compiler to CC (or cc by default) to ensure that the
architecture flag detection is done on the same compiler that is used to
compile libreadline. Without this libreadline will default to gcc, which
might not be the same as cc and also does not support -arch if it is the
real gcc. The Python extension module will typically be compiled by clang
on Darwin, which should be the default cc.

This follows @ohanar's suggestion in issue #28.
Remove unused imports and improve error message
Add workaround for clang on OS X 10.9.2

The latest clang from Xcode 5.1 treats unrecognized command-line options as
errors while the current Python CFLAGS variable on OS X 10.9.2 still contains
some unrecognized ones (e.g. -mno-fused-madd). See Xcode 5.1 Release Notes
(Compiler section) and http://stackoverflow.com/questions/22313407 for more
details.

This fix tests whether the actual compiler has an issue with the CFLAGS and
adds a flag (-Wno-error=unused-command-line-argument-hard-error-in-future)
that overrides the error behaviour, following the approach suggested in
http://stackoverflow.com/questions/724664. This addresses issue #35.
Update README on installation and versioning

Suggest the optimal way to install gnureadline to avoid problems.
Also correct the description of the module version numbering scheme.
Update Python module code

Use Python 3.x readline.c from hg 3.4 branch (changeset ID
89086:3110fb3095a2) while 2.x module stays the same. Patch
the file to rename the extension from readline to gnureadline.
Build against readline 6.3 (patch-level 3)

Use the latest and greatest readline, but only after the first
official patches have been released. Patch 002 is especially
helpful as it fixes a segfault when gnureadline is used with
IPython. This version of readline should support all future
versions of Darwin for now, so drop the 005a patch modification.
Prepare for release 6.3.3

Update changelog, use new package name and bump version number in setup.py.