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/Execute.py

@@ -0,0 +1,14 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import subprocess
+
+from beast.util import String
+
+def execute(args, include_errors=True, **kwds):
+    """Execute a shell command and return the value.  If args is a string,
+    it's split on spaces - if some of your arguments contain spaces, args should
+    instead be a list of arguments."""
+    if String.is_string(args):
+        args = args.split()
+    stderr = subprocess.STDOUT if include_errors else None
+    return subprocess.check_output(args, stderr=stderr, **kwds)