Reworked the calculation of new splice site positions.

src/Modules/Mutator.py:
- For deletions, position shifts are now active from the first position
  following the deletion. Previous behaviour was from (but not including)
  the first position of the deletion itself.
- Added method to check if the resulting shift gets smaller at some
  specific position: Mutator.shift_minus_at(position).
- The positions in the shift list are now interpreted as a shift from
  (and including) the listed position. Previous behaviour was following
  (and not including) the listed position.
- The Mutator.newSplice(sites) method has some additional logic. This
  fixes some bugs, including Trac bug #30.

src/tests/test_mutator.py:
- Written a lot of unit tests for the changes in Mutator.py.



git-svn-id: https://humgenprojects.lumc.nl/svn/mutalyzer/trunk@188 eb6bd6ab-9ccd-42b9-aceb-e2899b4a52f1