ENH: Filter low-mass features again after refining position.
BUG/TST: special case for topn=1
PERF: Memoize masks -- HUGE perf. boost for large masks.

dallan@dielectric-pc:~$ ipython test_mask_perf.ipy
10000 loops, best of 3: 136 µs per loop
dallan@dielectric-pc:~$ ipython test_mask_perf.ipy
1000000 loops, best of 3: 1.56 µs per loop
Merge pull request #23 from danielballan/fix-topn

Feature tests, fixes, and performance boosts.
BUG: minor remaining topn bug
Merge pull request #24 from danielballan/minor-topn-bug

BUG: minor remaining topn bug