prepare 1.9.0 release

[eli-darkly]

[1.9.0] - 2018-12-12

Added:

• The requestTransformer option allows you to customize the outgoing request in any way you like. (#28)
• It is now possible to specify the endpoint as either a URI or an OkHttp HttpUrl. (#29)
• The backoffResetThresholdMs option controls the new backoff behavior described below.
• Added Javadoc comments for all public members.

Changed:

• The exponential backoff behavior when a stream connection fails has changed as follows. Previously, the backoff delay would increase for each attempt if the connection could not be made at all, or if a read timeout happened; but if a connection was made and then an error (other than a timeout) occurred, the delay would be reset to the minimum value. Now, the delay is only reset if a stream connection is made and remains open for at least backoffResetThresholdMs milliseconds. The default for this value is one minute (60000).

[eli-darkly]

Note, we had not been maintaining a changelog for this repo before. I've added one, copying the release dates and notes from previous releases (although not all of those were documented).