Accepted only properly formatted version strings

for assignment operation.
Refactored IntegerGreaterThanOrEqualToZero

partial active pattern in order to simplify it a bit.
Ensured that the assign operation can handle

partial version strings.
Incremented patch version,

in order to indicate that checking for a properly parse version string
when assigning a version, constitutes a fix, in the sense that it's better
input validation.