Fix for 1.9, expose Response#offset

#5 describes how `String#[]` is broken under Ruby > 1.8. This is fixed to work in all versions.
#6 describes how `@client_time_receive` is not accessible and is an integer.

In addition to these fixes, I've also swapped to using `IO#select` to get a more accurate timestamp reading and fixed the "Originating Timestamp" field to be based off a float.

All tests pass on Rubies 1.8 .. 2.1 on my Ubuntu machine.