[#179] Add 'cancel' button

* Clean up files on 'onDestroy' callback
[#179] Add MD5 checksum property
[#179] Read & compare MD5 checksum

* Read hex hash from the json response
* Discard corrupted files
* Adjust layout
[#179] Use S3 ETag as a fallback

* Check first the json-encoded MD5, if exists
* Otherwise, fall back to S3's ETag header field
[#179] Update comment
i[#179] Avoid multiple instantiations on rotation
[#179] Update text
[#179] Remove unnecessary file
Merge pull request #183 from akvo/issue/179-apk-update

Issue/179 apk update