Added a Homebrew formula for installing libcpr on Mac OS X.

See: https://github.com/mxcl/homebrew/wiki/Formula-Cookbook
Fixed build errors due to included C++ code in <cpr/error.h>.
Added Homebrew installation instructions to the manual.
Added a MacPorts portfile for installing libcpr on Mac OS X.

See: http://guide.macports.org/chunked/development.html
See: http://guide.macports.org/chunked/reference.html