Changed count-source-lines script.

The script previously counted blank lines; the script now counts only nonblank
lines.

Previous command:

    cat foo.py | wc -l

New command:

    cat foo.py | sed '/^\s*$/d' | wc -l

Previous output:

    source_module_names: 901
    test_module_names:   1481

    source_lines:   116991
    test_lines:     78418

    total lines:    195409
    test-to-source ratio is 0.7 : 1

New output:

    source_module_names: 901
    test_module_names:   1481

    source_lines:   94387
    test_lines:     62335

    total lines:    156722
    test-to-source ratio is 0.7 : 1

CONCLUSION: line count drops from 195,409 to 156,722 when counting nonblank
lines only.

We'll use nonblank line counts in release notes and on the website.

NOTE: would be nice to break docstrings out of source line counts.