gh-103 : added role "DELETE_SECONDMENT"
gh-103 : deletion of secondments is now possible.
gh-103 : update, delete and proper activation of secodnments is now
possible.
gh-103 : finally new secondment is working flawlesly
gh-103 : extra tests have been added to prevent secondment addition when
there is an existing service allocation or disposal.
gh-103 : seconds management is now clear and does not allow secondmnets
to be added if the employee has a service allocation or disposal.
gh-103 : secondments for employees from other PYSDE once again working.
Merge pull request #111 from slavikos/gh-103

re-implementation of secondment management