Store cross compiled Ruby into seperate directories depending on HOST platform.

Also prefix the selection of rbconfig in config.yml with the corresponding
Ruby platform.

This allowes to use cross compiled Ruby-builds for multiple target
architectures on the same machine. This is especially usefull to compile
for Ruby-2.0 i386-mingw32 and x64-mingw32 platforms.
Add possibility to set per-platform cross_config_options.

This is especially useful to set distinct library paths for different
target platforms.
Use platform specific directories for packaging gem files.

Building gems for distinct platforms within one rake run conflicted
in the lib/ directory, because all binary extensions were stored
there and were copied from there to become packaged.
This resulted in storing a binary extension of a wrong platform in
a gem.

To avoid this, the files to package are now stored in a platform specific
stage directory and are used to package the specific gem.
Merge pull request #74 from larskanis/master

Add support for cross builds for multiple target platforms.