Add code coverage config and expand test suite

Adds .coveragerc configuration file to control coverage analysis settings.
Expands test suite with additional unit tests for AbstractProcessor,
SubjectMapper, CitationBuilder, LicenseProcessor, PIFinder, and
MetadataProcessor classes.

Updates README with comprehensive testing documentation, including
information about current code coverage (53%) and instructions for
running tests with coverage analysis.

.coveragerc

@@ -0,0 +1,23 @@
+[run]
+source = doi2dataset
+omit = 
+    */tests/*
+    */docs/*
+    setup.py
+    conf.py
+    __init__.py
+
+[report]
+exclude_lines =
+    pragma: no cover
+    def __repr__
+    if self.debug:
+    raise NotImplementedError
+    if __name__ == .__main__.:
+    pass
+    raise ImportError
+    except ImportError
+    def __str__
+
+[html]
+directory = htmlcov