units 0.8-2

• Names are preserved when doing unit conversions; #305 @billdenney

• Identical units will always divide (/) and allow integer division (%/%).
  And, inverse units will always be able to multiply; #310 @billdenney

• Compare units via ud_compare(), fixing inconsistent results for aliases
  and symbols; #339, #346, #347

• Fix units<-() to treat an empty unit the same as NULL; #332

• New as.data.frame() method for mixed_units; #309

• Use short paths for database loading on Windows to fix UTF-16 issues; #342

• Add example in the docs about reversing a ggplot2 units scale; #343

• Force storage mode to double; #344 addressing #324

• Fix units in transformed axis; #323

• Consider unitless as radians in trigonometric functions; #328