Security bug with threshold

[OleksandrShtepa]

There is a security bug in the library.
shamirs.shares(value, quantity, threshold)
actually works as:
shamirs.shares(value, quantity, threshold-1)
So shares() outputs unmodified value if threshold==2:

>>> shares(1234, quantity=3, threshold=2, modulus=10000)
[share(1, 1234, 10000), share(2, 1234, 10000), share(3, 1234, 10000)]
>>> interpolate(shares(1234, quantity=10, threshold=5)[:4])
1234

[lapets]

Thanks!