test cases for version filters ...

- tests all combinations of inferred, specified, and inherited version filters

Add support for DragonFlyBSD ...

- Switch from /bin/sh to /usr/bin/env bash
- Ignore temporary and test-build files (gitignore)
- Untrack changes to source/dub/version_.d
- Handling 'isDragonflybsd'
- Use 'tr -d' instead of 'tr --deleted' which works on Linux and BSD based platforms
- Fix FRONTEND = unbound variable (test/run-unittest.sh)

Remove all trailing whitespace ...

> sed 's/[ \t]*$//' -i **/*.d

fix OSX test ...

- libcrypto has an additional dependency on libz
- use non-existent libraries to avoid any pkg-config differences

Make the dependency test packages compile on Windows.

Fixed: --data=linker-files misses linker files specified in dub.json.

Add 'dub describe --data='.

Fix #560: --import-paths error when "configurations" section exists

Add dummy .d files to import paths for tests, so the directories will appear, as git ignores empty directories.

Add --import-paths and --string-import-paths as options for 'dub describe' instead of a new command. Add some scripts for testing the output of each, and a smoke test for 'dub describe'.