Try to fix knn test.
Add travis status to README
Merge pull request #171 from EasonLiao/master

Try to fix forest test.
Add refcnt to Tile and increase it when an expr reuse it. (like
regional_map). Before destroying tiles, the worker has to decrease
and check refcnt.
Dummy commit
Merge remote-tracking branch 'upstream/master'
Previous commit takes wrong tile_id(s) from the results.
'tile_id' in the results could be None. We need to check it.
Merge pull request #173 from fegin/master

Add refcnt to Tile and increase it when an expr (like regional_map) reuses it.