Added variable arguments length UDFs

@xlfunc
def my_udf(x, *y):
    ...

All attributes apply to each single argument passed, for example

@xlfunc
@xlarg("y", dims=2)
def my_udf(x, *y):
    ...

will ensure that each element of y is 2-dimensional, not y itself.

VBA7/64-bit PtrSafe bug fix in xlpython.bas

Fixed bugs relating to issues #12 and #14