adding DocTranslator contructor

To avoid the issue https://github.com/jterrace/sphinxtr/issues/29
line41,invisit_Textifself.verbatimisnotNone:AttributeError:DocTranslatorinstancehasnoattribute'verbatim'
Merge pull request #30 from emilopez/master

adding __init__ method to DocTranslator to avoid elatexpdf build error