Fix Grant McDermott's bugs

[kylebutts]

From @grantmcdermott

• Why load the datasets (df_hom and df_het) from did2s in the examples and README? Aren't they bundled with this package too?
• I think you can cut down a number of dependencies (and avoid some of the NSE / namespace clashes as we've seen here), by going fully native on the internal data.table code. LMK if that's something you want me to take a stab at, although I'm quite time constrained at present.