Accept files without translations headers

[Chouby]

Fixes #23

Our MO readers (both Dynamic and Full) are expecting translations headers to read the plural foms, but:

• WPML creates files without these headers at least to translate form labels in Gravity Forms.
• The MO file format description doesn't state if these headers are mandatrory or not.

This PR ensures that MO files without translations headers are correctly read and used.