Documentation for Microsoft compiler bugs
Support for MinGW

* Adds support for MinGW, a Linux-hosted cross compiler for Windows
  based on GCC.
Use _strtoui64 with Microsoft rather than strtoull
Use _strtoi64 with Microsoft rather than strtoll
Removed some conditional compilation for Microsoft compilers

* All of the code specific to Microsoft's compilers has been abstracted
  into functions and macros in Sawyer.C and Sawyer.h; none of the other
  code needs to have "#ifdef _MSC_VER".

* Windows-specific conditional compilation is still scattered around
  and protected by BOOST_WINDOWS. This code concerns itself with POSIX
  versus Windows APIs, not differences between compilers.
Changes so cmake files work with GCC, MinGW, and Visual Studio
Added DLL export/import markers

* Also fixed a few warnings
Added some notes about compiling for Microsoft Windows