#33 underscore_word() doesn't work for all caps, spaces, etc

[Maribel Sanchez]

underscore_word() doesn't work for all caps, words that
already have underscores (it adds extra underscore),
words with spaces, and I'm not sure how it's suppose to
run with numbers.

This is how it runs:
>>> underscore.mangle("Under_Scored_Word_")
'under__scored__word_'
>>> underscore.mangle("UNDER SCORE WORD")
'unde_r scor_e word'
>>> underscore.mangle("Under_3Scored")
'under_3_scored'
>>> underscore.mangle("3Upper 3Case")
'3_upper 3_case'
>>> underscore.mangle("under score word")
'under score word'