Implemented mail address class hierarchy:
- Created the base abstract class MailAddress as a root for mail address hierarchy;
- Implemented USMailAddress class supporting US mail address format;
- Covered MailAddress and USMailAddress classes by unit tests.
Created ForeignMailAddress and unit test placeholder classes.
Fixed CodeAnalyzer warnings.
Created the base abstract overloaded method ToString in MailAddress class.
Restored all unit tests in Eithery.Core project.