Add basic interoperability with other Observable implementations

[martijnwalraven]

Closes #149.

[stubailo]

@martijnwalraven hey, I don't know how this happened, but it looks like you skipped the PR checklist, which included "add this to the changelog". Can you please add a changelog entry about this?

[stubailo]

@martijnwalraven also, the CI tests didn't pass on this PR.

[martijnwalraven]

@stubailo: Sorry I missed the changelog. I've added it to the 0.3.5 section because it was included in that release.

The tests passed on the CI but I missed the linter errors. Thanks for fixing these.

[stubailo]

Yeah no problem! Next time just wait until Travis is done before merging.

[martijnwalraven]

I actually did look at the CI results but figured there was something else wrong because the tests themselves passed. Should have taken a closer look at the logs and spotted the linter errors.