Version v0.1.7: serde and rustc-serialize made optional features (sti…

…ll enabled by default)

Version v0.1.6: implements `Ord` on symbols

Version v0.1.4: symbols are now garbage-collected

Version v0.1.3: implements `Encodable` and fixes validation bug

Version v0.1.2: better implementation of Clone, Eq and Hash traits

Version v0.1.1: fixes Send + Sync implementation