Look for custom processors in `lib/paperclip`

... instead of `lib` of `lib/paperclip_processors`.
Fixes thoughtbot/paperclip#1534
Re-add lib/paperclip_processors for custom processors.

Also, add integration tests for custom processor definition and unit
test for Paperclip.load_processor (thoughtbot/paperclip#1535).
Reflect custom processor changes on README.md.

(thoughtbot/paperclip#1535)
Add Simplified Chinese (zh-CN) translation.
Add Traditional Chinese (zh-TW) translation.
Update attachment.rb

Add explicit unlink to intermediate files from processors.
Make id partitioning for string similar to integer
Merge pull request #1640 from JuanitoFatas/locale-zh-TW

Add Traditional Chinese (zh-TW) translation.
Merge pull request #1639 from JuanitoFatas/locale-zh-CN

Add Simplified Chinese (zh-CN) translation.
Pass options through to the underlying adapter.

Column-specific options can be specified by using its name,
for example:

```
t.attachment :avatar, created_at: { index: true }
```