Merge pull request #16 from danielballan/master

update readme
Merge pull request #17 from danielballan/master

more readme edits
Merge pull request #18 from danielballan/subpx

ENH/TST: Improve subpixel precision and verify it with tests.
Merge pull request #19 from danielballan/sub-clean

CLN: Remove print statements.
Merge pull request #20 from danielballan/topn

ENH: Support topn keyword, like Crocker/Grier pickN.