Pre-release alpha v0.2.1

Added

• Environmental variable DTNOOPENMP will cause the datatable to be built without OpenMP support.
• If d0 is a DataTable, then d1 = DataTable(d0) will create its shallow copy.
• In addition to LLVM4 environmental variable, datatable will now also look for the llvm4 folder
  within the package's directory.
• Getter df.internal.rowindex allows access to the RowIndex on the DataTable (for inspection / reuse).
• Implemented statistics min, max, mean, stdev, countna for numeric and boolean columns.
• A framework for computing and storin g per-column summary statistics.
• sys.getsizeof(dt) can now be used to query the size of the datatable in memory.
• This CHANGELOG file.

Fixed

• Filter function when applied to a view DataTable now produces correct result.