Refactoring a bunch of stuff, adding lots of sanity checks and more type-checking around user-input, handle more failure cases gracefully.

1. Use Jsdoc to generate documentation around our SDK classes. This PR looks really big, but 90% of it is just new files (CSS, font, html files) to support the static docs. Also had to update a lot of comments through the SDK to generate the docs.
2. For the Readme section on config inputs, I added a type column to specify the expected input type. The SDK has a helper function that parses the config input object, validates the keys and input types against the DEFAULT config object.
3. Refactored a lot of static strings into Constants.js
4. We can simplify the input validation by leveraging the type library. Before we would do something like this: if (var === null || type(var) !== 'string'), but type(null) returns 'null', so you can simply just do a type(var) !== 'string' and that will also check for nulls and undefined
5. Following 4, we make the SDK more defensive by aggressively checking the input, for null, for types, for valid values
6. If we are using a generated deviceId, append an 'R' to the end, to make JS SDK consistent with our iOS and android SDKs
7. Since we validate eventProperties on init, we might as well validate userProperties.
8. Handle loading of saved events from localStorage where client is using a broken JSON library, so deserialization into an array fails.
9. As suggested by Ryan, we add names to all of the anonymous functions.
   Before: var Amplitude.prototype.func = function(blah)
   After: var Amplitude.prototype.func = function func(blah)
10. Now save initial_UTM parameters too. Following the style of the referrer, we stop sending UTM params on every event since they are a user property. Instead we send them once per session (basically during the init of the SDK), and save the values to sessionStorage. If we see values already existing in sessionStorage, then we know that they were already sent. Refactored code so that UTM and referrer processing can share the same function
11. Moved a bunch of helper methods into the utils module
12. user_id should not default to device_id if null
13. Added tests for 2, 6, 7, 10

@paladin8 you can skip everything in the documentation folder

really solid changes, only worry is that it is pretty big so we just want to make sure we manually test enough and that upgrades work, etc. lgtm from a code perspective though!