0.4.0 Faster indexing

• Raise the limit of number of fields (previously 256 fields) (@fulmicoton)
• Removed u32 fields. They are replaced by u64 and i64 fields (#65) (@fulmicoton)
• Optimized skip in SegmentPostings (#130) (@lnicola)
• Replacing rustc_serialize by serde. Kudos to @KodrAus and @lnicola
• Using error-chain (@KodrAus)
• QueryParser: (@fulmicoton)
  • Explicit error returned when searched for a term that is not indexed
  • Searching for a int term via the query parser was broken (age:1)
  • Searching for a non-indexed field returns an explicit Error
  • Phrase query for non-tokenized field are not tokenized by the query parser.
• Faster/Better indexing (@fulmicoton)
  • using murmurhash2
  • faster merging
  • more memory efficient fast field writer (@lnicola )
  • better handling of collisions
  • lesser memory usage
• Added API, most notably to iterate over ranges of terms (@fulmicoton)
• Bugfix that was preventing to unmap segment files, on index drop (@fulmicoton)
• Made the doc! macro public (@fulmicoton)
• Added an alternative implementation of the streaming dictionary (@fulmicoton)