[clang-tidy] misc-braces-around-statements.ShortStatementLines option

Add option ShortStatementLines to trigger this check only if the statement spans
over at least a given number of lines.

Modifications from the original patch:
  merged test/clang-tidy/misc-braces-around-statements-always.cpp into
  test/clang-tidy/misc-braces-around-statements.cpp and removed unnecessary
  CHECK-NOTs from the tests.

http://reviews.llvm.org/D5642

Patch by Marek Kurdej!