Better type support

• rescale(I(x), ...) and rescale_mid(I(x), ...) return I(x) unaltered
  (@teunbrand, #403).
• Add a rescale method for difftime objects (#382)
• Add better support for difftime objects. label_timespan() adds
  functionality for adding correct unit suffix to timespan data,
  breaks_timespan() adds functionality for finding pleasant breakpoints across
  the various bases in time units, while transform_timespan() wraps it all
  together and provides an alternative to transform_hms() (#212)

Ranges

• train_continuous() coerces new to numeric before calculating range
  (@teunbrand, #369).
• Training on factor data no longer sorts the range after multiple training
  passes if the new fct argument of train_discrete() is used (#383)
• The DiscreteRange class now keeps track on whether it has been trained on
  factor data.
• Attempt to make the sort behavior of the range consistent for character
  vectors during training. Mixing of character and factor data will make the
  factor level order take precedence no matter if the training starts with a
  charactor vector

Transformations

• Transformation function have been renamed to transform_*-prefixed names
  instead of *_trans-suffixed names. This allows for a better tab-completion
  search of transformations. The S3 class of transformations has been
  renamed from "trans" to "transform". new_transform() replaces
  trans_new() and trim_to_domain() replaces trans_range(). All old
  functions are kept for posterity.
• transform_sqrt() no longer returns an inverse for values outside of its
  domain (#214)
• Add an inverse (area) hyperbolic sine transformation transform_asinh(),
  which provides a logarithm-like transformation of a space, but which
  accommodates negative values (#297)
• Correct the domain calculation for transform_compose() (@mjskay, #408).
• Transformation objects can optionally include the derivatives of the transform
  and the inverse transform (@mjskay, #322).

Misc

• The scale_cut argument in number() now works as advertised for values
  below the lowest cut value (#346)
• scale_cut will now choose a more appropriate cut in the case of the first
  cut creating an irrational-ish number.
• Added a new option to the style_positive argument in label_*() functions.
  Setting this to "space" will add a figure space in front of the number to
  make it easier to align positive and negative values as figure space takes up
  the same amount of space as - (#366)
• label_dollar() has been superseeded by label_currency() for clarity (#344)
• Palette functions now have the pal_-prefix. The old _pal-suffixed versions
  are kept for backward compatibility.