Adding first test and some helper functions

Fixes #2
Refactored calc_sha1 and validate_sha1

Fixes #2
Adding more tests

Fixes #2
Some cleanup and fixes after review

Fixes #2
Use webmock for testing the download functions

Fixes #2
Add tests for eval_file

Fixes #2
Make sure we cleanup temp files/dirs

Fixes #2
Some changes after review

Fixes #2
Improve download spec test

Fixes #2
Add more tests

Fixes #2
minor fixup after review

Fixes #2