Remove HTTP method check from HTTPServerRequest._parse_body.

The semantics of a body are not well-defined for some methods,
but if a client sends a body with the correct content-type,
we might as well parse it no matter what the method.

Closes #1011.
Improve performance of HEAD requests on large static files.

StaticFileHandler would previously read the entire file and throw it
away just to compute its length; now it uses get_content_size()
instead().

Added extra validation in tests by performing both GET and HEAD
versions of all requests and ensuring the content headers match.

Closes #988.
Stream large response bodies from StaticFileHandler.

The get_content interface is still synchronous so it's not a complete
solution, but this will keep the server from buffering the whole file in
memory while writing it out to the client.

Return Futures from the wsgi interface, allowing @gen.coroutine to be
used there in limited circumstances.