Allow passing of encoding to load_dotenv and dotenv_values (Pr/144)

[theskumar]

closes #144

[coveralls]

Coverage increased (+0.04%) to 87.838% when pulling d4d5dc1 on pr/144 into 2767b35 on master.

[coveralls]

Coverage increased (+0.04%) to 88.542% when pulling e9c1954 on pr/144 into 98e4eee on master.

[bbc2]

I think it would be good to add that the default encoding changes from platform-dependent to UTF-8, especially since it might break on some platforms where UTF-8 isn't the default. What do you think?

[trim21]

Could use locale.getpreferredencoding() as default value. This will keep behavior stay same as before, because it's default encoding when we call open.

[uranusjr]

You can simply pass encoding=None to let io.open infer the user’s default environment encoding.

[theskumar]

Passing encoding=None sounds like the way to go, breaking the existing API doesn't sound like a good idea.

[theskumar]

I've updated the encoding to be platform independent now, it's not a breaking change now. cc: @bbc2