Don't warn for missing appname if version is NULL

* if version is NULL we don't need to be warned that it will be ignored
* this was a hurried addition in 8428ce0301 on top of Gabor Csardi's
  a4ef238573
Switch tests to tests/testthat

* this is the new recommended location, allowing tests to be installed or not
* see https://github.com/hadley/testthat#integration-with-r-cmd-check
Fix tests on Windows by wrapping baseline paths in file_path

* use internal file_path function to normalise baseline paths in the same
  way as the xxx_dir() functions
* using file_path avoids having to use:
  normalizePath(file.path(), mustWork=FALSE)
Merge pull request #10 from jefferis/fix/wintests

Fix tests on Windows
Merge pull request #9 from jefferis/fix/appname-warning

Don't warn for missing appname if version is NULL
Clarify the main purpose of the os argument
Merge pull request #12 from jefferis/feature/clarify-os-arg-docs

Clarify the main purpose of the os argument
Depend on methods for sake of Rscript + S4

* closes #11
Merge pull request #13 from jefferis/fix/depend-methods

Depend on methods for sake of Rscript + S4
Add NEWS file
Version 0.3
docs: caveats section notes race conditions

* also move details re CRAN policies out of description and into a new
  caveats section lower down.
Merge pull request #14 from jefferis/feature/cran-0.3

Feature/cran 0.3
Merge pull request #15 from jefferis/feature/package-docs-caveats

docs: caveats section notes race conditions
Merge pull request #7 from krlmlr/setup

Setup
Merge remote-tracking branch 'hadley/master' into develop