cli: Add a fallback to calculate the SHA1 if it is not present in the cache

[sschuberth]

@pombredanne I've updated the PR with a commit that addresses your TODO of writing a LicenseRef for licenses not known to SPDX.

[pombredanne]

@sschuberth Thanks!
I am sorry that you have to resort to this... I think instead that I may prefer always adding the checksums (even if the file is empty) and instead add a new file-level flag `is_empty1 that would state the file is essentially empty junk: e.g. really empty, size is 0, or composed only of a single character (e.g. ----- or zeroes) or composed only of white spaces (spaces, tabs, lf, etc).... Let me think about this overnight: this is small and easy change and might be much cleaner.

[sschuberth]

I agree that adding hashes even for empty files is more consistent. However, you also told me I shouldn't rely on cache internals, and maybe at some point the info does not expose the SHA1 anymore, so having a fallback to calculate it might make sense anyway.

WRT storing info about empty files, maybe instead of introducing a whole new flag, consider adding another type in filetype.py?

[pombredanne]

The info will always expose a sha1 (the fact that it is also used as a cache key is the only "internal" implementation part).

And you got it right: that would be a new flag in filetype.py

[pombredanne]

@sschuberth I am merging this for now so this is not in your way and I created #461 to update the behavior