Fix compiler warnings

Some of the compiler warnings started to bug me, so I tried to fix them. They're usually harmless, but sometimes they are not, as I will explain.

Currently, [these](https://gist.github.com/4678787) are the warnings that are thrown during a python2.7 build of matplotlib (`clang` on a mac is used here)

After this patch is applied, [these](https://gist.github.com/4678809) are the warnings that are still left. The first four are due to a 'bug' in the python API; treating string literals as `char *` rather than `const char *`. There is a patch [upstream](http://bugs.python.org/issue9369) for it, but I'm not sure if it will be back-ported to python 2.7.4.

The last bunch of `Tcl`/`Tk` warnings are because the compiler is being handed both a `-c` flag and a `-framework T{cl,k}` flag, which are meant solely for the compilation and linking steps, respectively. To fix them, one would need to separate out the compile and link steps, but I don't know how to do that with `distutils`, and they only appear on the mac. Linux and windows compilers do not support the `-framework` flag.

I have tested this patch and I get a successful build with both Apple's builds of gcc and clang. All the tests pass locally with python 2.7. It would be nice to have these tested with python3, and a linux flavour of compiler.

I also fixed a typo in a comment.