Fix crash in dxr/build.py when source tree is empty.
Remove all .pyc files on build.

This fixes a bunch of ImportErrors when switching back and forth between the es and master branches. The git-ignored .pyc files are left alone, and Python happily loads them in various cases.
Don't have dxr-build crash when the tree is empty. Close #347.
Merge master to get *.pyc deletion and to merge test_empty while I still remember what it's for.