Make `query` take a string instead of an object

[rauchg]

• Adapt the README accordingly
• Add query tests
• Make sure sid and transport get added to the query by appending to the string using encodeURIComponent
• Make sure transport code gets updated
• If an object is passed, stringify it right away for BC.
• If the URL contains a query (as a result of util.parseUri), use it as opts.query (include tests for this as well)

[rauchg]

Tests go in the engine.io repo.