Fix mixed indentation style and follow PEP-8 style guidelines

I noticed that project had mixed indentation. Some files had tabs, other spaces. Sometimes mixed even within single files. Now it consistently uses 4-spaces. By the way i fixed numerous PEP-8 violations as it increases code readability.

There are only 3 flake warnings:
```
$ flake8 pyexcelerate
pyexcelerate/vfs.py:12:17: F821 undefined name 'Directory'
pyexcelerate/tests/test_Workbook.py:128:9: F841 local variable 'expected' is assigned to but never used
pyexcelerate/tests/test_Workbook.py:129:9: F841 local variable 'got' is assigned to but never used
```

I didn't fixed them because they indicate that there could be some issues:
# `vsf` module is probably broken
# `test_workbook.test_number_precision` case probably lacks one of intended asserts

Full list of fixes in commit messages.