More helpful validation

[Rich-Harris]

Trying to clear out the easy-to-tackle stuff from the issues list. This closes #166, with some analysis of helpers and methods (and namespace, while I was at it):

• Event handlers (on:click='foo()') are checked to ensure that foo is a valid method. Valid methods are set, fire and anything custom. this.* and event.* are also permitted
• If foo doesn't exist on methods but it does exist on helpers, Svelte asks if it was put in the wrong place
• We do some sanity checking on helpers to make sure that they're in the right place:
  • If a helper uses this.get('x') — common mistake — it errors, tells you to pass it in as an argument instead (we could also do this for computed values)
  • If it otherwise uses this, it errors, telling you that helper functions must be pure and have no access to the component instance, and suggesting that it belongs on methods
  • If the function length is zero and arguments is not used, it warns you that helpers should be pure
• Invalid namespaces cause an error. Svelte will use fuzzy matching to try and guess what you meant, to help in case of typos like https instead of http.