0.2.0

[andytimm]

Merging this back in now that all the issues are addressed.

Changes:

• Added use of a non-central t-distribution in the functions that use simulation. This makes a really minor difference outside tiny-n cases, but does improve results in those cases. Thanks to Martijn for the initial implementation I worked from!
• retro_design is soft-depricated in favor of retro_design_closed_form, to make what the function does much clearer.
• The original retrodesign function was ported with only minor changes, which meant it could give misleading results if a vector was passed for A. This is now resolved- a package should anticipate common input errors.
• Names of outputs are now consistent across all functions.
• Vignette was updated for above changes.

[andytimm]

Pushed this to cran just now, time to see what if anything they don't like. But will merge back into master because from my perspective it's done