-VisitableDocValueFields API for persisted DV field list
-making dv configs overridable at field level
-enabling on the fly/runtime un inverting of doc values
-few UT updates
removed analyzers (these are now built as needed through config)
removed html chacter filter (now built as needed through config)
added missing license header
changed constructor signature of filters that cannot return errors
filter constructors that can have errors, now have Must variant which panics
change cdl2 tokenizer into filter (should only see lower-case input)
new top level index api, closes#5
refactored index tests to not rely directly on analyzers
moved query objects to top-level
new top level search api, closes#12
top score collector allows skipping results
index mapping supports _all by default, closes#3 and closes#6
index mapping supports disabled sections, closes#7
new http sub package with reusable http.Handler's, closes#22
Sreekanth Sivasankaran