ycm_core should be ycm_core.pyd on Windows
Removing ycm_core.dll as allowed name

I'm pretty sure that allowing ycm_core.dll was wrong. Python docs state that the
extension for compiled Python modules on Widows is "pyd"; see this link for more
details: http://docs.python.org/2/faq/windows.html#is-a-pyd-file-the-same-as-a-dll

Key quote from the page:
"Of course, foo.pyd is required if you want to say import foo."

CMakeLists.txt has been previously updated to produce a ycm_core.pyd on Windows,
not a ycm_core.dll.