Check for an integer NA in the R index vector for a factor.

In addition to using an index of 0 in a factor to indicate missing data, some
index vectors end up with an integer NA value.
Avoid use of an explicit uint16 for converting the refs in a PooledDataArray

The practice now is to use POOLED_DATA_VEC_REF_CONVERTER
New version of ModelMatrix

The current approach is to use

mf = ModelFrame(f::Formula, d::AbstractDataFrame)

to create the model frame, followed by

mm = ModelMatrix(mf)

to create the model matrix.  The response is retrieved with

y = model_response(mf)

Still work to be done.
Changes that should have gone in the previous commit but somehow didn't.
Change import of names from the Sort module
Import functions and types from Base.Sort for which methods will be added.
Better show for IndexedVectors.
Use complete_cases for na_omit operation (thanks to Tom Short).
Merge remote-tracking branch 'origin/master'
Consistency in converters for PooledDataArray refs.
Fix #245, rbind stack overflow.
Small fix to enable zero-row DataFrames
Small fix to allow zero-row SubDataFrames
Merge pull request #248 from dreiss-isb/patch-1

Small fix to re-enable zero-row DataFrames
Merge pull request #249 from dreiss-isb/patch-2

Small fix to re-enable zero-row SubDataFrames
Small fix: re-enable zero-length PooledDataArrays

There may be other places in DataFrames which need this update too, but I haven't looked for them.
Added zero-length PooledDataArray test
Added zero-row DataFrame/SubDataFrame tests
Merge pull request #252 from dreiss-isb/patch-5

Added zero-row DataFrame/SubDataFrame tests
Merge pull request #250 from dreiss-isb/patch-3

Small fix: re-enable zero-length PooledDataArrays