Merge pull request #56 from yorkie/master

lexer: support exponential notation
Merge pull request #57 from yorkie/master

lexer: fix exponential notation of integer
add support for optional parens in param-less function definitions
rename "ret" to "return" again
add scanning of type token
add luna_type_node_t
add parsing of type node
add array examples
remove NEWLINE token

too annoying to work with, I would rather use
something other than space for slot delimiters etc
add conditionals.luna example
remove a context()
add todo
lexer: fix 1e-1 in expoential notation