features: add optional source_url_checksum to check for corrupted downloads.

source_url_checksum is of the form:
<hasher-name>:<hash-data>

e.g.
 md5:d44b5dddebd8a7a7309aea6c36fda117
 sha256:898388b2a876c10be5325e1a8bb46a0484ba2ae1478b1cdf796866426065754c

the list of valid hasher-names is anything supported by the hashlib std python module.
examples: add source_url_checksum example
Merge pull request #7 from hwaf/wip/md5-checkum

Wip/md5 checkum