Fixing some problems with makeProfileGraph

All arguments are now optional.

I was having problems with importing gprof2dot:
```
from external.gprof2dot import gprof2dot
```
works on Pharos with python 2.6
```
from external import gprof2dot
```
works on my Mac with python 2.7. There should be a better way of handling it.

Author: keceli