Uses python-shell-interpreter instead of hardcoded "python"

[furkanusta]

Fixes #10

Although the fix is general and should work for ipython as well, I didn't test ipython as I only use python3 currently. (It was working for ipython at the time I submitted this PR before)

[anachronic]

I took a look at this and it seems to work ok as long as you don't set more than one single-string argument at a time. For example: if python-shell-interpreter-args is "--arg1 --arg2", importmagic will fail. I see the need for this and I'll implement it probably this weekend.

The readme you provided is kind of misleading and leads to fail when trying to use ipython. From what i understand, python-shell-interpreter-args is supposed to be a single string and not a list of strings. This is what causing importmagic to fail, because epc expects a list of strings.

As I said, I'll do it this weekend and I'll also provide some testing for it. Thanks for the contribution, though. I'll be closing this once everything is fixed.

[anachronic]

#15 is opened for officially closing this issue. If you could try it out (branch devel), please leave comments in #14.