do not throw error on empty or missing line data

instead fill as unknown
improve middleware response behavior

* do not render html for json requests, instead pass forward to the next middleware.

* render response with status code 500.
remove renderJson method from api

that opens space for other modules down the error stack to capture the error and send custom response.
fix tests due changes in module's api
improve error message 'header' parsing

* show all the error message
* supports jade errors & etc.
api features json response and next middleware

based on req.accept returns either json or html representation of the error.

* fixed tests
* responses render with express 3 with status code 500
* if request doesn't accepts json/html then next middleware is invoked.
add support to disable html and/or json responses

* updated readme
* added tests
better errors with multiline header

* improved header parsing
* moved error details in its own section
* added util/testServer helper for /bigerror
Merge pull request #9 from outbounder/fix_and_improvements

One fix and couple improvements