Public APIs refactor with top-level dotenv constant

[pulyaevskiy]

This ended up being a bit more than I expected. Addresses #13 .

• Added top-level dotenv constant to avoid namespace collisions;
• Declared previous APIs deprecated;
• Updated tests, documentation and examples.

Additionally:

• Implemented "integration" tests (loading env from files) ( #9 )
• Replaced stderr output with private library logger, to give users control over output of this library.
• New API does not provide write access to the env map for better encapsulation.

This is a big change, however existing APIs should still work (I added a test case for this). If this gets merged it should probably go in 0.2.0 with deprecation notice for old APIs to be removed in 0.3.0.

I can submit separate PR for removing old APIs.

Please let me know your thoughts!

[pulyaevskiy]

Friendly reminder @mockturtl

[mockturtl]

@pulyaevskiy Thanks for your patience, again. I've been tied up with other things.

Looking over it now.

[mockturtl]

Note to self: @Deprecated, @deprecated

[mockturtl]

New API does not provide write access to the env map for better encapsulation.

I agree this is a good idea. I really want to keep the top-level getter spelled env, though.

See #15.

[pulyaevskiy]

I think it's probably better to imitate the Map interface here and spell it containsKey. Thoughts?

It makes sense to me. I'll change it.

Re: sdk constraint.

Looks like it's only in pubspec.lock file, probably result of pub upgrade. I think guidelines recommend to "gitignore" lock file for libraries (here). Should I just remove the whole file from git tracking and add it to .gitignore?