Improvements to scons build for beast.

* Common code extracted to Python directories.
* Read ~/.scons file for scons environment defaults.
* Override scons settings with shell environment variables.
* New "tags" for debug, nodebug, optimize, nooptimize builds.
* Universal platform detection.
* Default value of environment variables set through prefix dictionaries.
* Check for correct Boost value and fail otherwise.
* Extract git describe --tags into a preprocesor variable, -DTIP_BRANCH
* More colors - blue for unchanged defaults, green for changed defaults, red for error.
* Contain unit tests for non-obvious stuff.
* Check to see that boost libraries have been built.
* Right now, we accept both .dylib and .a versions but it'd be easy to enforce .a only.

python/beast/util/Dict.py

@@ -0,0 +1,17 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+def compose(*dicts):
+    result = {}
+    for d in dicts:
+        result.update(**d)
+    return result
+
+def get_items_with_prefix(key, mapping):
+    """Get all elements from the mapping whose keys are a prefix of the given
+    key, sorted by increasing key length."""
+    for k, v in sorted(mapping.items()):
+        if key.startswith(k):
+            yield v
+
+def compose_prefix_dicts(key, mapping):
+    return compose(*get_items_with_prefix(key, mapping))