Added in-browser tests

[fazo96]

Closes #5

This PR includes commits from #1 and #2 so they should be merged before looking at this.

Changes summary:

• adjusted all tests to use mocha instead of jest, because jest tests can't run in the browser
• added Karma to run the mocha tests in the browser

Some notes:

• karma does not run the multi-node tests (the ones involving more than one ChluIPFS node) because connecting directly two IPFS nodes in the browser is tricky. It does run all the other tests though including the ones using a real IPFS node.
• karma does not run in CI because I can only get it to run "hands free" using chrome which is not available in travis. Firefox is available but I can't figure out how to make it work. I'll try to fix this

Luckily almost no changes were needed to get ChluIPFS to run in the browser.