Speed up anyna, allna. Reinstate nas tests.
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Merge pull request #109 from JuliaStats/testna

Speed up anyna, allna. Reinstate NAs tests.
fix ref/value problem in unique for pooled data
handle final NA special case in unique
add tests for unique on pooled data (integers and strings)
fix unique() for DataArrays with only final NA
remove non-PooledDataArray test