Make Lemur suitable as a library

[jeroenvandijk]

Guided by #23 I've continued the work of @jimdowning.

The code should be suitable now to use a library, although it is a bit ugly still due to the use of one dynamic var. This was the easiest option for now. I hope to do some more iterations later on. I say "should" because I still need to test it from a project. The tests I could run without the need for AWS access were succeeding, but not sure about the others.

For short-term future work I see the following:

• Have library functions that take the context as an argument
• Move dsl functions in lemure.core to a separate namespace

Happy to hear your first feedback on these changes

[mlimotte]

i don't think you need to mention all the explicit keys here.

[jeroenvandijk]

True. I'll remove them.

[jeroenvandijk]

@mlimotte I've made corrections based on your suggestions.

[leon-barrett]

nit: this line's indentation of [core] is inconsistent with the line below

[leon-barrett]

Aside from me being picky about indentation, this seems great. @aykuznetsova said she might be able to test this out with our use cases, so let's wait a bit to merge for her verification.

[jeroenvandijk]

I've created a test environment to test the new changes https://github.com/jeroenvandijk/lemur-test-ground

The old functionality seems to be okay. The new functionality is not complete yet and needs some more iterations, or at least some investigation.

[mlimotte]

We should update CHANGES.txt also.

And, eventually, the version number and the README-- but maybe we do those post-commit.