Use 64bits VM

To be able to build 64bits binaries for Windows we need 64bits
compilers that are not provided by Ubuntu.

These compilers requires 64bits host OS, so we need to adjust the VM
for this.
Install Ruby 2.0.0 in the VM

To target 64bits Windows, we need Ruby 2.0.0 as is the only version of
Ruby RubyInstaller provides binaries for.
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.
Merge pull request #2 from luislavena/support-x64-builds

Support Windows fat 1.8/1.9/2.0 binary builds and x64 builds