Massive Makefile clean up!

- "make all" builds all combinations of configurations
- Binaries are built to "bin"
- (For convenience, the release interpreter also goes in the top level dir)
- Libraries are built to "lib"

This will also make it easier to support building and testing other
configurations like Nan tagging versus union, computed goto, etc.

.gitignore

@@ -1,19 +1,12 @@
-# Intermediate files directory.
+# Build outputs.
+bin/
+lib/
+wren
+
+# Intermediate files.
 build/
 .sass-cache/
 
-# Built files at the top level.
-wren
-wrend
-wren-cpp
-libwren.a
-libwrend.a
-libwren.so
-libwrend.so
-libwren.dylib
-libwrend.dylib
-libwren-cpp.a
-
 # XCode user-specific stuff.
 xcuserdata/
 *.xccheckout