Fix bug with Project struct

[zahiar]

The IsPrivate was incorrectly named, it should be Is_private to match the name returned by the API. This lead to this value always remaining false, as the decoder was unable to match the is_private key from the API response with IsPrivate in the Project struct.

Fixes #157

Test Output

$ BITBUCKET_TEST_USERNAME=? BITBUCKET_TEST_PASSWORD=? BITBUCKET_TEST_OWNER=? go test -v ./tests/project_test.go
=== RUN   TestProjectCreate_isPrivateTrue
--- PASS: TestProjectCreate_isPrivateTrue (1.56s)
=== RUN   TestProjectCreate_isPrivateFalse
--- PASS: TestProjectCreate_isPrivateFalse (0.92s)
PASS
ok      command-line-arguments  2.946s

[ktrysmt]

Thanks a lot!