Merge pull request #255 from timema/master

fix PooledDataArray ERROR: no method sort(KeyIterator{Dict{Int64,Uint16}},)
Minor bug fixes

* has() -> haskey() (there are likely more of these)
* typo in test/pooleddataarray.jl
Replace deprecated calls to has() with contains()
Modified formula.jl to handle ~ Days + (1+Days|Subject)
Replace has() calls; add collect()'s
Merge pull request #256 from timema/master

fixes load_df when deserializing stored dataframe
Fix one of the has->haskey changes
Convert R factors to new-style PooledDataArrays with short refs