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/Git.py

@@ -0,0 +1,9 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import os
+
+from beast.util import Execute
+from beast.util import String
+
+def describe(**kwds):
+    return String.single_line(Execute.execute('git describe --tags', **kwds))