[clang-tidy] Support initializer_list in google-explicit-constructor check

Summary:
According to the Google C++ Style Guide, constructors taking a single
std::initializer_list<> should not be marked explicit.

This change also changes the messages according to conventions used in Clang
diagnostics: no capitalization of the first letter, no trailing dot.

Reviewers: djasper

Reviewed By: djasper

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D6427