+ .gitattributes
Merged code from #4 with changes:
* moved compile-time validation into IValidateContractUsageAttribute;
* reduced validation aggressiveness (now raises a warning and ignores invalid annotations, instead of raising errors);
* standardized name of new attribute.
bumped version; +GlobalAssemblyInfo for simpler assembly versioning.
fix: build error if annotated property has no accessible setter.
fixed: build errors if type is from an outside assembly or generic type is unknown at compile-time (re: #4).
added *.pdb file to NuGet package.
cleanup.
updated Json.NET and NUnit.
Fixed unit tests.
Optimizations: avoid LINQ Union in favour of Concat when we don't need duplicate checks; rm unneeded methods.
Optimization: use the faster default serializer (unless operating in partial trust).
updated release notes.
fix: allow [NotEmpty] on non-generic enumerations.