Run R CMD check without warnings

[rcannood]

I've been working on allowing the R CMD check to complete without warnings.

Changelog:

• Added a .Rbuildignore, otherwise the build check complains about various directories being included in the package
• Added a .travis.yml file, which allows you to automatically build and check your package on travis-ci.org. Check it out! It's really cool! This is the travis page for dynverse/SLICE. It still fails on every build, because warnings generated by the build check are treated as errors in the build.
• slice.R: added 'do.plot' arguments to several functions -- I want to be able to turn off the plotting

[rcannood]

I have just pushed a few more changes. You can see on travis-ci.org that the package is now nicely able to compile without any warnings.

• Fixed all the warnings and most of the notes that build checking produces
• Removed all non-default packages from Depends: to Imports:, and used roxygen's @import and @importFrom notation
• Use match.arg instead of pmatch to match arguments
• Fixed S4 documentation
• Added a skeleton for the demo documentation and the dataset documentation ─ better documentation definitely still needs to be added
• Resaved the data with compression

[rcannood]

Hello @xu-lab @mzguo @minzheguo,

Have you had a chance to look at this pull request yet?

Kind regards,
Robrecht

[rcannood]

Closing this pull request.