Use relative paths on files

[BanzaiMan]

Allow relative paths to be passed.

This effectively reverts #45.

I do not quite remember the rationale for #45, to be honest, but I believe that some relative paths didn't work so well before. This PR adds -l instead, which might be a good solution to the problem.

[BanzaiMan]

This PR might annul the existing caches with absolute paths, so we should test that.

[BanzaiMan]

I think the issue #45 attempts to solve is cache: bundler and friends, where relative paths names when ./vendor/cache didn't work on the bash version.

[native-api]

LGTM.

-l seems not really necessary since we compare files in the same environment but I guess it won't hurt -- and the output would look better with the paths looking as the user specified.

[native-api]

On closer inspection:

travis_build stock logic that uses ~ for cache paths will become broken since Bash won't expand it in $(checksum_checker) invocation:

$ paths="~ C:\\"
$ md5deep64 ${paths}
C:\Users\Sasha\~: No such file or directory
C:\: Is a directory

[native-api]

The changes also need to be duplicated into Ruby version since it's used in preference to Bash version if RVM is available.