Basic implementation of sending If-Modified-Since per request.
IfModifiedSince: this patch passes all existing tests.
Merge branch 'master' into IfModifiedSince
IfModifiedSince: Added ability to recover the requested headers in MockRemoteServer.
IfModifiedSince test case written and passing.
IfModifiedSince: improve comments.
Handle MockRemoteServer.Headers errors correctly in IfModifiedSince
branch.
Merge branch 'master' into IfModifiedSince

Conflicts:
	test/fetcher_test.go
Merge pull request #78 from iParadigms/IfModifiedSince

If modified since