Cleanup in Token class.

Made constructors explicit to avoid implicit conversions. Added const
where appropriate. Replaced isType with getType. Removed assignment
operator since Token should be immutable. The main function now uses the
Galaxy namespace by default.
Added Lexer class.
Rename TokenType EOF/EOL

These types conflict with existing macro in cstdio.
Add precedence for Token.
Fix TokenType in Lexer (EOF to END_FILE).
Add TokenTest and LexerTest.