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
@Fishrock123 Express.js lower-casing all header names in `req.headers` object, so it must be lower-cased. Package user can write header name in any case. Probably I should add a test for case-insensitivity of header name option?