Download and setup mingw-w64 compilers

These compilers will open the room for cross-compilation of newer
versions of Ruby, like 2.0.0

- Download the latest stable/release binaries (4.7.2) used by
RubyInstaller too and setup inside ~/mingw

- Append the `bin` directories of mingw-w64 to the PATH.
Task to cross-compile all Rubies

Instead of delaying the compilation of the cross-version of Ruby, we
trigger all the compilation at once.

- Use Ubuntu's GCC to compile 1.9.3 to match the GCC version used in
Windows (DevKit)

We do this to avoid cross-compilation of some gems detect
characteristics of mingw-w64 headers that will not be present on
Windows version of 1.9.3

This happen when using mingw-w64 to cross-compile projects like
EventMachine that uses some settings of mingw-w64 headers not present
in Windows counterpart for Ruby 1.9.3

Doing this we reduce the chances of messing things up when
cross-compiling gems.

- Turn debugging off for Ruby 2.0

For some reason, Ruby 2.0 enables it's debugging flags when
cross-compiled, resulting in massive binaries for all extensions
compiled with it.

While this has been reported to Ruby-Core developers, this still hasn't
been fixed yet.

Instead, reduce the debugging symbols to the minimum to allow gdb
remain usable.
Remove cross-compilation from specific tasks

Prepare cross-compiled Rubies in one place and use them.
Use rake-compiler 0.9.1
Adjust platforms used by sample gem

Support:

- x86-mingw32 (i386-mingw32)
- x86-mswin32-60 (i386-mswin32 - backward compatibility)
- x64-mingw32 (x64-mingw32 - newer 64bits)
Include Ruby 2.0.0 on fat-binaries too

Now that we have 1.8.7, 1.9.3 and 2.0.0, we can safely build
fat-binaries.
JRuby: update and lock version of RubyGems

JRuby 1.7.4 do not have RubyGems 1.8.25 with it, so an update is
required.