Aggregation reducers

Whats new

• The aggregation reducers such as Avg, Sum and Count was introduces along with Chain and GroupBy reducers.
• Entry got some new mthods
  • Getters Field(name string), FloatField(name string)
  • Setters SetField(name string, value string), SetFloatField(name string, value float64), SetUintField(name string, uint64)
  • Utility methods Merge(entry *Entry), FieldsHash(fields []string), Partial(fields []string)

Backward incompatibilities

• All functions deals with *Entry instead of Entry
• MapReduce returns chan *Entry instead of chan interface{} and all reducers accept output channel as chan *Entry
• Entry is a struct, not a map[string]string anymore and has two constructors NewEntry that accepts Fields and NewEmptyEntry
• Entry.Get was renamed to Entry.Field