0.5.0

• Breaking change: open_subkey now opens a key with readonly permissions.
  Use create_subkey or open_subkey_with_flags to open with read-write permissions.
• Breaking change: features transactions and serialization-serde are now disabled by default.
• Breaking change: serialization now uses serde instead of rustc-serialize.
• winapi updated to 0.3.
• Documentation fixes (#14)