Improve coverage for fs operations
Improve coverage for fs operations

* TestCopyDir: add subdirectory to the test case; extend test to handle
  multiple files in different directories.

* Add tests for error conditions in CopyDir: source directory cannot be
  read; destination directory cannot be read; files cannot be read.

* Add tests for error conditions in IsRegular.

* Add tests for error conditions in IsDir.

This addresses issue #41.

Signed-off-by: Marcelo E. Magallon <56cce93673b8e0eba950695f805812ddc478ba9f@gmail.com>

ibrasho-forks/dep,golang/dep,golang/dep,golang/dep,ibrasho-forks/dep