Don't propagate IOException when GET for /CrumbIssuer fails.

[hamaron]

post() and post_xml() method in JenkinsHttpClient.java has a bug where we always get IOException when CrumbIssuer is not set up in a Jenkins server.
Specifically, the following code always throws IOException before doing POST.

Crumb crumb = get("/crumbIssuer", Crumb.class);

#26 resolved the Crumb issue for users who set up CrumbIssuer, but it is not always the case.

To support the other users who have not used the CrumbIssuer, we should modify this.

[hamaron]

Checked in a patch and created a pull request for this: #39

[hamaron]

Rebased the branch, considering the feedback given in #39.
The new PR is #110

[khmarbaise]

Has been integrated into release 0.3.3