Split parsing into several files.
Added additional unit tests.
Moved unit tests from lib.rs to headers.rs.

Now parsing is done with 3 phases:
1) HeadersFinishAnalyzer - that detects "\r\n" sequence at start line.
2) HeaderNameParser - that parse and trim header name before COLON.
3) HeaderValueParser - that parse header value. This is a temporary implementation of value parser.