Allow to search messages without using a time frame but at least one filtering is required
Remove un-necessary comment and exception

No need to raise an exception, in the worst case the user will just retrieve the first or
last ``rows_per_page`` messages.
Some argument validation for the grep method.
Merge pull request #44 from fedora-infra/no_timestamp_query

Allow to search messages without using a time frame but at least one fil...