switch singular GitHub struct fields to pointers

Like a51d6b4303, this change makes me sad, mainly because it is a
breaking change for all clients, and makes common tasks like reading
data out of structs slightly more work, with no direct benefit. Notably,
developers will need to make sure and check for nil values before trying
to dereference these pointers.  Sadly, the change is still necessary, as
is more fully explained in issue #19.  We can make the nil pointer
checks a little easier by adding some Get* funcs like goprotobuf does.

I spent a lot of time over the last few weeks exploring this change
(switching fields to pointers) versus the much larger change of using
protocol buffers for all GitHub data types.  While the goprotobuf
library is very mature and feature-rich (it's used heavily inside of
Google), it's the wrong tool for this task, since we're not actually
using the proto wire format.  While it does address the immediate
concern in #19, it makes way too many other things terribly awkward.

One of the biggest drawbacks of this change is that it will make the
string output from fmt.Printf("%v") next to useless, since all pointer
values are displayed as their memory address.  To handle that, I'll be
writing a custom String() function for these structs that is heavily
inspired by goprotobuf and internals from go's fmt package.
add Stringify function

This provides somewhat reasonable string representations of GitHub
structs.  This is specifically designed for this library and takes a
number of shortcuts where it can, so is not suitable as a general
purpose solution.  That said, I am exporting this function because it is
useful for things like printing out slices of GitHub structs, as can be
seen in examples/example.go.

I am certainly open to suggestions for what exactly the stringified
output should look like.  Currently, I think I've found a reasonable
compromise between fmt's "%v" and "%s" output.