Outsource request auth to oauth library

The oauth library is capable of generating http.Client objects
which automatically wrap requests with the appropriate authen-
tication header. clever-go now relies on this library instead
of setting the header on each request. To cover the case of
APIKey authentication, a BasicAuthTransport struct is added
with methods RoundTrip and Client. When/if APIKey authenticat-
ion is removed from the system, the BasicAuthTransport struct
should be removed.
Add tests for auth request headers

clever_test.go: New testing around clever-go's auth request headers. Added
three new tests: (1) TestEmptyAuthError passes if clever.Query, when calle
d with empty Auth credentials, raises an error. (2) TestBearerAuth passes
if the http client created by oauth.Transport successfully wraps requests
with the appropriate authorization header. This test can fail in two ways,
depending on whether the authorization header was missing or incorrect.
(3) TestBasicAuth is the same as TestBearerAuth but for Basic auth.

mock.go: newMock now has extra optional parameters which are pointers to
type map[string][]string. None of the old tests pass anything into these
parameters. The new tests TestBearerAuth and TestBasicAuth use this parm-
eter to collect the most recent request header after a call to
clever.Query.
Clarify test failure codes

Clarified the failure codes for the new tests; for example
showing expected and actual error codes when querying with
empty credentials.
Switch travis.yaml over to Go 1.2
Merge pull request #4 from Clever/API64-auth-to-httpclient

API64 - Now using oauth's Transport.client() to automatically wrap requests with the appropriate auth headers.