nicholascarey/loomeR: Bug fixes, unit testing, preparation for CRAN submission

A couple of important bug fixes related to padding videos. Also lots of minor code changes related to passing CRAN checks, and also added unit testing code.

v0.2.2 Changes

• FIX: making animation from a diameter_model would fail when padded.
• FIX: padding would sometimes fail because number of frames being added was not an integer.
• citation("loomeR") should return a citation.
• Added unit testing. Most code now covered by testthat checks.
• Added numerous console messages and input checks to help with testing.
• Moved internal functions to separate script and removed unused ones.
• Minor changes to pass build checks and prepare for CRAN submission.
• Minor changes to get_alt console print out.
• Revised README to separate code chunks and add citation() code.
• Added some URLs to DESCRIPTION.
• Added macOS to Travis build checks.
• Added Appveyor Windows build checks.