Change version datatype

Hi,

I'd like to change the version datatype from `uint64_t` to `int64_t`, because:
- There is no `scanUnsignedLongLong` method on `NSScanner` before Mavericks (Yesterday I ran into a bug "unrecognized selector sent to.." on Mountain Lion. Although I set the deployment target to 10.8, Xcode did not warn me about that method not being available on 10.8.. Worked fine on my 10.10 machine but not on my customers 10.8 iMac)
- The SQLite datatype INTEGER is signed
- 9.2 x 10^18 (INT64_MAX) should be more than enough for version numbers

I also changed the tests and the README. Everything passing and working fine both on Yosemite and Mountain Lion.

I hope my changes are not too intrusive? I also added `copy` to `NSString*` property declarations as recommended by Apple.

If you do not like this approach at all, we could also do something like this:
```
int64_t ullValue;
[scanner scanLongLong:&ullValue];
return (uint64_t)ullValue;
```
However I am not sure if that's a good idea.
I'd prefer using int64_t.

Regards, Clemens