Added support for passing session ID using HTTP header

These changes add new option `headerName` that can be used to optional passing session ID using custom HTTP header.

Already discussed this issue in #77
@dougwilson but if I convert header name to lower case in the constructor, package user will see lower-cased HTTP header in response, is this ok? I beleive package user should see exactly what he passed to the options.

Probably there should be two variables, one for `getHeader` function and one for passing unchanged header name back in the response?