Reword for clarity.

No change in the message, but hopefully the text is clearer than before.
Add Simon to the list of authors.
Mark files excluded from source distribution.
Make gitinfo return empty dictionary with no git.

Also cache git data so it gets executed only once.
Make libdiffpy.so the default build target.

This ensures "scons" will not build all other targets.
Add "sdist" scons target for source tarball.
Move feature macros to features.hpp.

version.hpp needs to be set at sdist time, whereas features are
determined at compile time from possibly a source distribution.
Set default build mode to "fast".