Clean up fetch-sequence command line option

1) Rename 'fetch-sequence' to 'fetch'. "fetch-sequence" can be keeped as deprecated for some time to preserve compatiblity. Reason: fetch-sequence fetches complete resource images, but not sequence only.

Probably we need additional JIRA issues for some future release not to forget to remove deprecated code;

2) Make database names used in command line short and case insensitive. E.g. genbank but not "NCBI GenBank (DNA sequence)". Probably we should support both short and long names here.

Document all available database both in --help & manual

3) Replace "Nothing has been found" with "Not found" for files content that failed to download