[YoutubeDL] Escape non-ASCII characters in URLs

urllib chokes on URLs with non-ASCII characters (see http://bugs.python.org/issue3991)
Working around by replacing request's original URL with escaped one
[YoutubeDL/utils] Clarify rationale for URL escaping in comment, move escape routines to utils and add some tests
[utils] Remove debug garbage
Merge remote-tracking branch 'dstftw/escape-non-ascii-in-urls'

Conflicts:
	test/test_utils.py