BLD: Add useful shortcuts to Makefile
Add some shortcuts to make it easier to develop with pandas.

now there's a set of commands in the `Makefile` in the top-level pandas directory with the following functionality
- `make clean` will delete the `build` and `dist` directories + all `*.pyc` and `*.so` files
- `make clean_pyc` just removes `*.pyc` files
- `make build` will build extensions inplace
- `make develop` will install `pandas` in your environment but will place a link to the dev dir so that you can make changes and they will show up immediately
- `make doc` will build the documentation from scratch (erases generated and build directories)
