Print errors for failed tests by default

Errors can optionally be silenced by passing -q
Add output to subsets examples.
Merge pull request #641 from astrieanna/add_output

Add output to subsets docs examples
tweak show(GroupedDataFrame) to match other show methods
Merge pull request #642 from JuliaStats/showgrouped

tweak show(GroupedDataFrame) to match other show methods
exported HOFs all do thunk

Some functions have a method whose final arg is a function.
The do block idiom doesn't work for those methods, so alternate methods
were added (exported functions with a single function arg only).
Merge pull request #643 from JuliaStats/bythunk

exported HOFs all do thunk
Allow invalid identifiers