Preliminary implementation of 100 Continue support

Add a boolean to the Request type indicating if an expectation check
has been specified by the user, using setExpectContinue.

Alter sendRequest to check for this bit and manually receive a status
line and headers first before sending body.
Handle 417 Expectation Failed

Push (what turns out to be a real) response back to the input stream
so that the error response will be available in the readResponse
function.
Improve documentation for setExpectContinue
Merge branch '100-continue'