Fix Ruby API

[andreaslillebo]

• The Ruby API makefile now compiles without any "hacks"
• The ruby version and path is now automatically detected
• The SWIG interface file for the Ruby API now includes modules from the SWIG library, instead of defining custom fragments inline
• The ruby sample code has been updated with clearer variable names to make it easier for people to understand and customize
• The readme has been updated to reflect that the Ruby API now is working (the content was mostly copied from the Python API readme)
• These changes also fixes #83, where the API would crash on non-ascii characters

[lluisp]

Thanks!