More aniseed.core functions!

Added to aniseed.core...

• merge, merge various maps together, just like Clojure.
• kv-pairs, turn a table into a sequential table of key/value tuples.
• map-indexed, basically kv-pairs and then map. Just like Clojure.
• with-out-str, capture anything printed by pr or println within the callback.
• get, get-in, assoc and assoc-in. Just like Clojure but assoc will mutate.
• butlast, everything but the last item in a table.
• empty?, returns true if what you give it has a count of 0.
• mapcat, like Clojure.

Misc

• I alias aniseed.core as a now all over the place since I use it so much.
• Use println internally over the plain Lua print so you can capture output of core functions with with-out-str.
• More tests!