Simplified getting name of provider

Looking into how python -m faker > docs.txt works to generate documentation, noticed that getting the name of a provider inside `Documentor.get_provider_name()` (a static method) was a bit complicated. Therefore this simplification to help maintenance...

Background: the following works interactively (with or without this PR applied):

```
>>> from faker import Faker
>>> from faker.documentor import Documentor
>>> fake = Faker('nl_NL')
>>> for p in fake.get_providers():
...   print p.__provider__, '\t', Documentor.get_provider_name(p)
... 
user_agent 	user_agent
ssn 	ssn
phone_number 	phone_number
python 	python
profile 	profile
person 	person
misc 	misc
lorem 	lorem
job 	job
internet 	internet
file 	file
date_time 	date_time
credit_card 	credit_card
company 	company
color 	color
address 	address
base 	base
>>> 
```