Make pyyaml a requirement

[perrotuerto]

Ruby i18n lib supports YAML by default. For this lib pyyaml is an extra requirement. I suggest to make it a requirement. With this, issues like #7, #9, #19 and #26 could be avoided.

In addition, I will suggest to load .yaml and .yml by default, instead of using i18n.set('filename_format'...), since Ruby lib also does that by default.

[sectasy0]

Seems this package is no longer maintained by the author, check out this library https://github.com/sectasy0/pyi18n

[Myzel394]

@sectasy0 Unfortunately this package does not support namespaces :/