Additional admin warnings

[mpsonntag]

This PR adds additional admin warnings when processing a DOI request. Some of these changes require changes in the libgin library merged into master.

• admin warnings are added if a reftype other than isSupplementTo is used. Closes Allow all reftypes through the validator #73.
• a warning is added, if the ID type of a reference is not available. Changes in libgin now exclude such entries from the XML file and the warning points this out to the admin. Closes Filter empty RelatedIdentifier #76.
• warnings are added, if an authorID has duplicate entries, if the authorID type cannot be identified, or if the ID type is known, but the ID value is missing. Changes in libgin now exclude authorID entries that only provide the ID type but not the ID value. Closes Filter empty datacite (ORC)ID fields #77.
• a warning is added, if the resourceType is not Dataset.
• when writing the HTML Author block, now there is no longer an authorID link created if no valid SchemeURI has been
  identified to avoid in-site broken links.
• when registration errors occur, e.g. if the License file could not be found, when required fields are missing in the datacite.yaml file or when unsupported datacite values have been used, the collected error messages are now shown to the user as a formatted list. Closes Itemize invalid DOI request messages #94.
• adds tests for added functionality.

The PR also updates the README file and adds CI badges and links.

[coveralls]

Pull Request Test Coverage Report for Build 698787967

• 59 of 84 (70.24%) changed or added relevant lines in 3 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+3.4%) to 20.718%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/gindoid/dataset.go	0	7	0.0%
cmd/gindoid/validation.go	47	65	72.31%

Files with Coverage Reduction	New Missed Lines	%
cmd/gindoid/validation.go	3	76.53%

Totals
Change from base Build 646077620:	3.4%
Covered Lines:	381
Relevant Lines:	1839

---

💛 - Coveralls