Interfaces for Issue #16:

* added missing method setSourceStream to interface Lexer
* changed description of getNextToken to never returning null
* add TokenType NOT_A_TOKEN and EOF for token error handling and stream end marking
* add descriptions for tokens
* changed Token interface methods getLoc and getCoc to better readable getLine and getColumn
Interfaces for #16:

* add setter for Lexer and ReportLog
* defined ReportLog for convenient error reporting