Add constants for build status

[strarsis]

Adding constants for the build status codes to docker-hub-api would make code more self-documentary:

[...]
if(build.status == dockerHubAPI.BUILD_STATUS_SUCCEEDED) {
[...]
if(build.status == dockerHubAPI.BUILD_STATUS_FAILED) {
[...]

[RyanTheAllmighty]

Yeah there are some changes I want to make for version 1.0.0 as I'm not entirely happy with the way things are at the moment with this package. While it works, it needs some work done and this is definitely a step towards a better library.

[strarsis]

There are 2 more status codes: 0 for queued 3 for building