Fix empty string in attribute description in ldap

- if description is empty string, work with it like with null
- use in resource and group
Merge pull request #231 from stavamichal/ldap_fix_empty_string_in_description

Fix empty string in attribute description in ldap
GUI: fixed behavior on member's status change

- Cell displaying membership status will now refresh itself,
  so we don't have to refresh whole table.

- Unified MembershipStatus widget implementation and fixed
  events chain, that was broken on member's detail tab.

- Remember checkbox state and display loading state in table
  when tab is refreshed using refresh button in top-right corner.

- Pass checkbox state to "list all" callback, when user check it
  first and then load vo members.
Merge pull request #232 from zlamalp/members

GUI: fixed behavior on member's status change
GUI: preparation for custom error handling

 - Standard error widget can now be generated by method in UiElements.
 - Call URL and posted object can now be injected to PerunError and by this
   passed to custom handling in method onError(PerunError error) of callback.
- Modified JsonClient and JsonPostClient to reflect this error.
- Source formatting needed because of formatting change in sources.
GUI: custom error handling for AddMember call

 - Custom error handling for AddMember callback for
   AlreadyMemberException, WrongAttributeValueException
   and WrongReferenceAttributeValueException.
 - Fixed mistaken error.getType() for error.getName().
GUI: custom error handling for more calls

- Added custom error handling for calls AssignGroupToResources
  and AssignGroupsToResource.
- Fixed custom error texts.