feat($log): add $log.debug()

[nlaplante]

New debug() method with suppressable output via
$logProvider.debugEnabled()

Closes #1592

[lanterndev]

Might be worth looking at @Wizek's implementation in https://groups.google.com/forum/#!msg/angular/vgMF3i3Uq2Y/q1fY_iIvkhUJ if you haven't seen it yet?

[Wizek]

Hi @nlaplante! Do you miss anything in the implementation @Skivvies linked? If so let me know so I can improve it.
If not, do you think I should send it in as pull request no. 61?

p.s. Thanks @Skivvies for mentioning me. :)

[pkozlowski-opensource]

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[nlaplante]

@pkozlowski-opensource I already signed the CLA a while back. Do you need me to do it again?

[pkozlowski-opensource]

@nlaplante no, if it is signed there is no need to sign again. Thank you!

[vojtajina]

Thank you @nlaplante, merged as 9e991dd.

@Wizek @Skivvies the implementation from mailing list is nice, but I don't think we want this in the core. You can easily implement it on the top of current $log.