Added check for curl.

Bash scripting is a lot harder than I thought; I basically
hacked around for two-ish hours just trying to figure out
the behavior of the unit testing framework (shunit2), how
to output values from functions (really, one does that through
echo statements), and how to avoid using straightforward constructs
in favor of less intuitive constructs, all because I'm using "set -e".
Added function that outputs OS X version.

Easier than I thought because I'd done my homework on this
ahead of time.
Added function to output OS X major version.

Pretty simple addition to an existing function. I'd consider
refactoring, but...

1) It's an easy snippet to add.
2) "cut" requires a "file", and that's easier to pipe in
3) I see no point in learning sed or awk for something I can do
   already in a single line of bash that took me ~30s to figure out.
4) At this stage in the installation, I'm assuming I only have bash;
   otherwise I'd use Python or Ruby.

Frustrated with bash scripting right now, because I suck at it, having
avoided it for so long.
Added function that gets compiler tools pkg URL.

First use of case statement and if statement in bash scripting. Glad
not to have screwed it up. A few long lines here; may want to
revise.
Revised function that gets compiler tools URL.

Made it easy to modify the URLs for each compiler, since the
URLs will inevitably change.