Fixed clang completeAt

Added additional parameter to session: clang_language
  it's needed by clang to pass correct -x parameter. I think it's good idea to get language from editor
Implemented finding beginning of identifier we want to complete at
  it's needed to meet requirement of http://clang.llvm.org/doxygen/group__CINDEX__CODE__COMPLET.html#ga50fedfa85d8d1517363952f2e10aa3bf

TODO: sublime text has problems with completing destructors because it assumes that identifier must start with letter or underscore while destructors are starting with ~.