Maintenance release

• Minor tweaks for compatibility with purrr 0.2.2
• as_Composite_table.data.table() converts data.tables to Composite tables,
  based on separators in the column names (e.g fruit.apple, fruit.banana)
• Improved README / vignette / documentation
• Changed how as.data.table.Composite_table() constructs column names. The new
  format is colname.multiname instead of multiname.colname. This is to emulate
  the behaviour of base::merge()
• flip_names() can flip names and multinames of a composite table (at the cost
  of reordering the columns)