Do not chdir() to phakefile directory

This is done in order to preserve relative path passed as arguments, as
they're relative to the current working directory.

Consider the directory structure:
```
project/
|- demo/
|   \- example.txt
\- Phakefile.php
```

Consider I have a task named `test` which accepts a single argument `file`:
- If I'm in the directory `project/`, I expect to be able to run `phake test file=demo/example.txt`
- If I'm in the directory `project/demo`, I expect to be able to run `phake test file=example.txt`

Open for discussion:
- What was the original rational for changing dirs? (What about rake / make?)
- Is this considered a BC break, and if so, how should this be handled?