feat(serializer): adds property mapping wildcard behavior

Also refactors tests, moving serializer related test to a separate file.
test(common): adds $send state propagation tests
fix(serializer): fixes wildcard mapping for decoding and adds missing test
chore: updates inflector
feature(relations.belongs_to): Improves the way relation works using mappings.

Makes the relation store foreign key in host object when it is serialized.

Deprecates usage of the inline option, now the relation will use both modes by
default.

Closes #82
bumps v0.16.1