add test test_double_can_be_spied
more tests for double_can_be_spied
try to implement spied double just with spy_into

but in vail. currently spy doesn't wrap methods added after spy_into
remove verbose parentheses
delegate stub feature to spy
declare define_wrapper method as a required method of SpyMixin
don't compare with block attached to a received_message so far
correct test: mistyped count_received method
make SpyBase instead of SpyMixin and Stubber

SpyBase abstracts common behavior more than SpyMixin.
Now SpyBase stubs method instead of Stubber to reduce modules to prepend.
discard stubber and with_stubber

now spy is responsible for stubbing
avoid warning when stubbing and spying a method
write document about spying stubbed methods
reorder the document

"Stub Methods of a Spy" is more relevant to "Spy on a Object" than "Spy on Instances of a Class"
double delegates received* methods of spy to its @spy
document about double's spy feature
forgot to delegate received_messages metod to @spy
breakline
fix breaklines
Merge pull request #21 from igrep/spy-into-double

Spy into double