CRAN v0.2.1

A small performance and methods release. Some issues with truncated
distributions have been fixed, and some more distribution methods have been
added which improve performance of common tasks.

New features

Probability distributions

• Added dist_missing() for representing unknown or missing (NA) distributions.

Improvements

• Documentation improvements.
• Added cdf() method for dist_sample() which uses the emperical cdf.
• dist_mixture() now preserves dimnames() if all distributions have the same
  dimnames().
• Added density() and generate() methods for sample distributions.
• Added skewness() method for dist_sample().
• Improved performance for truncated Normal and sample distributions (#49).
• Improved vectorisation of distribution methods.

Bug fixes

• Fixed issue with computing the median of dist_truncated() distributions.
• Fixed format method for dist_truncated() distributions with no upper or
  lower limit.
• Fixed issue with naming objects giving an invalid structure. It now
  gives an informative error (#23).
• Fixed documentation for Negative Binomial distribution (#46).