Fix default behavior of VectorField
Fix default behavior of VectorField

Per django documentation, best practice is to make changes to the __init__ method
only when setting hard defaults, or introducing new parameters.  Looking at the
original hard-set defaults, db_index and null have been removed from __init__
to allow the user to control the values for those params.  All other hard-set
params will remain, and require corresponding del entries in the deconstruct method
to allow django's migrations to work in 1.7+.

gsong/djorm-ext-pgfulltext,linuxlewis/djorm-ext-pgfulltext,barseghyanartur/djorm-ext-pgfulltext,megahall/djorm-ext-pgfulltext,asmedeus/djorm-ext-pgfulltext,pombredanne/djorm-ext-pgfulltext