Fix for BSON timestamp serialization issue for C extension

[renctan]

Hi Kyle,

Please accept my pull request for the fix. Here's the background:

Problem:

The problem was that the serialization/deserialization code assumes that the seconds/increment is always Fixnum. Ruby documentation defines Fixnum as a number that fits the native machine word size minus 2 bits (one is used as signed bit, and another apparently for differentiating between an address and raw integer value). So, for 32bit machines, the maximum Fixnum is just 1073741823.

Fix:

I just changed the code to use a type conversion macro that will perform checks if the numeral is a Fixnum or Bignum. I have also modified the seconds/increment to be unsigned. Although the BSON spec does not explicitly mention whether it is signed or not, the mongo source code defines the i and secs member of OpTime as unsigned so I made the changes to make the serializer consistent with the mongo definition.

Reference:

This has been extremely helpful in analyzing the c extension code (especially on ch2) which you might also find useful:
http://i.loveruby.net/ja/rhg/book/

They also have a partially translated version available here:
http://rhg.rubyforge.org/

Thanks!