Make UTF-8 detection more robust.

If the 1st line of the Python is blank/empty, the function assumes that the encoding is 'ascii' and
doesn't try for the 2nd line.
Add one more assertion to a test about COVERAGE_PROCESS_START.
Merged in rogerjhu/coverage.py (pull request #19)

Make UTF-8 detection more robust.
Improve and more fully test the first-blank-line decoding fix.  Thanks, Roger Hu.
Get some more diagnostic output on Travis.