Fixed #52 -- Add URL encoding to otpauth URL

Google Authenticator gave an error when scanning the code using the
previous format. I added url quoting as well as the optional `issuer`
URL parameter.

Modified otpauth label format. Changed from `Site Name (username)` to
`Site Name:username`, which appears to be the most common format, although
both would probably be valid.

Modified test cases to reflect the changes made in `QRGeneratorView`
and `get_otpauth_url` (otpauth label and issuer parameter)
Standardized oauth URL geneator argument names

The revised method uses the names as defined in the specs, so it becomes
easier to identify them. Extra tests have been added to verify that the
generated URLs are correct.
Fire signal when user is verified

A signal will be fired when a user is verified. This would allow extra
customization like notifying the user when a backup device was used. See
the added documentation for an example implementation.
Fixed Travis install commands ordering

It was installing the package before installing Django. The package
triggers installation of the latest (stable) Django release. However
the test defines another version of Django, causing a re-install of
the specified version. By installing the correct version of Django
prior, only a single installation is required.
Enabled Twilio in the example app settings
Fixed #54 -- Mitigate voicemail hack

See also [this blogpost][1] which uses the victim's voicemail
service to access OTP tokens. When the call goes to voicemail, the
tokens can be access through a vulnerable voicemail service (e.g.
requiring no PIN). Then the attacker will have access to the tokens
and is able to login. When requiring interaction (e.g. pressing a
button), this type of attack can be disarmed. Major services like
Google and Yahoo are still vulnerable to this type of attack.

[1]: http://shubh.am/how-i-bypassed-2-factor-authentication-on-google-yahoo-linkedin-and-many-others/
Fixed #55 -- Use two_factor:login instead of LOGIN_URL
Added info for translators on twilio locale support
Added new languages supported by Twilio
Fixed code style (PEP8)
Translation for Brazilian Portuguese
Merge branch 'topic/pt-br_translation'

Conflicts:
	tests/tests.py
	two_factor/utils.py