Pigar

- Fixed IndexError.

- Fixed AttributeError.
- Fixed PEP8 warning.

- Removed useless code.
- Cache modules, to avoid duplication of inspection.

Thank [spacewander](https://github.com/spacewander) for the following contributions:

- Fixed error for Python 2.7.6.
- Fixed error when using './xxx' as relative path.
- Support `importlib.import_module` and `__import__`.

- Sort files, make comments more clearer.
- Fixed grammar, to make the README clearer. Thank [roryokane](https://github.com/roryokane) and [flyingfisch](https://github.com/flyingfisch).
- Make it work with `python -m pigar`. Thank [lilydjwg](https://github.com/lilydjwg).
- Fixed the pep8 warnings: [15](https://github.com/damnever/pigar/pull/15).
- Make output more clearer: [12](https://github.com/damnever/pigar/issues/12).
- Fixed UnicodeDecodeError for Python 3.

- If requirements file is overwritten over, show difference between old and new.
- Adjust the structure of the code.

- Fixed typo.
- Follow symlinks.