Fixes compilation error using libboost_1_50

Problem spotted in https://github.com/moses-smt/mosesdecoder/issues/32 
fixed.

According to the Boost doc, nbestPath.parent_path().filename() 
returns a path object, to get the correspondent std::string 
representation, one must call one of the methods listed in:
http://www.boost.org/doc/libs/1_53_0/libs/filesystem/doc/reference.html#path-native-format-observers

native() is supposed to return the path in the specific OS
path format (using backslashes for Windows). Anyway, since we 
are considering only the filename here, the result is the same.
Merge pull request #33 from jpuigcerver/patch-1

Fixes compilation error using libboost_1_50
Merge github.com:moses-smt/mosesdecoder into weight-new
eclipse
move RedVoc() as method in PDTimp class. Ready for getting rid of static variable
no leak message due to static variable in binary phrase table. Doesn't actually solve the mem leak though
get rid of locking altogether. PDTimp already has separated source & target vocab variable. Use those instead
eclipse
merge