Wrap README.md to 80 columns
Wrap INSTALL to 80 columns
Wrap CONTRIBUTING.md to 80 columns
Merge pull request #2595 from drbrain/80_column_README

Wrap README, etc. to 80 columns
Rename 'struct RBasic' to 'struct MRBasic'

This will help with embedding MRuby inside Ruby.
Rename 'struct RObject' to 'struct MRObject'

This will help with embedding MRuby inside Ruby.
Rename 'struct RString' to 'struct MRString'

This will help with embedding MRuby inside Ruby.
Rename RSTRING macros to MRSTRING

This will help with embedding MRuby inside Ruby.