code: public IssueCategory createCategory(IssueCategory category) throws RedmineException {
  		if (category.getProject() == null
  				|| category.getProject().getId() == null) {
  			throw new IllegalArgumentException(
  					"IssueCategory must contain an existing project");
          }
  
  		return transport.addChildEntry(Project.class, category.getProject()
  				.getId().toString(), category);
      }
docs: creates a new {@link IssueCategory} for the {@link Project} contained. <br/>
  Pre-condition: the attribute {@link Project} for the {@link IssueCategory} must
  not be null!
  
  @param category the {@link IssueCategory}. Must contain a {@link Project}.
  @return the new {@link IssueCategory} created by Redmine
  @throws IllegalArgumentException thrown in case the category does not contain a project.
  @throws RedmineAuthenticationException  thrown in case something went wrong while trying to login
  @throws RedmineException         thrown in case something went wrong in Redmine
  @throws NotFoundException        thrown in case an object can not be found