Remove unneeded import statements
Use a slice instead of a list

By replacing the container/list with a slice we can make use of
sort.Sort and subslicing to get our top-N termfacet results.
Merge branch 'termfacetrefactor' of github.com:Shugyousha/bleve into Shugyousha-termfacetrefactor
fix slice allocation to be size 0 cap len(fb.termsCount)
Merge branch 'Shugyousha-termfacetrefactor'