[log] inferred caller is wrong

[sdt]

When using the debug, info, etc. functions from the nagare.log module (SVN r223), the inferred caller of log records is these functions themselves.

This means the %(filename)s, %(funcName)s, %(module)s formatter arguments do not resolve to the correct source. For instance %(module)s resolves to 'nagare.log'.

Logger.findCaller rewinds the stack until it finds the standard logging module (itself) using _srcfile, which is only used for that purpose. Nagare may hijack it for the formatting functions to work correctly.

[apoirier]

(In [5ce74c46debed50fb3c21c29cc954c858c08f722]) Fixes #50