Use IOException in Callback failure method.
Merge pull request #923 from square/jw/failure-io

Use IOException in Callback failure method.
Update the language level to Java 7.
Merge pull request #935 from square/jw/seven

Update the language level to Java 7.
Fix two problems with the window size.

Each peer has a window size. The peer's window size controls how much
data we can send the peer, and our window size controls how much data
the peer can send us. We had these reversed in SpdyStream, so if the
peer specified a non-default window size we wouldn't send the required
WINDOW_UPDATE events when we needed to.

The connection has its own window size. But unlike the stream window
sizes, this needs to be manually adjusted with a windowUpdate frame
when the connection is opened.

Closes https://github.com/square/okhttp/issues/933