FIX: class_exts should return empty set, not empty dict
CLN/PERF: Do not import matplotlib at the module level.
ENH: In notebook, default repr of Frame is PNG.
Merge pull request #62 from danielballan/no-module-mpl

CLN/PERF: Do not import matplotlib at the module level.
Merge pull request #61 from danielballan/exts-set

FIX: class_exts should return empty set, not empty dict
Merge pull request #63 from danielballan/rich-display

ENH: In notebook, default repr of Frame is PNG.