Reacted to aspnet/Razor#215 changes.
Add out-of-order HTML attributes to MVC tag helper sample
- these are rendered in the same, non-alphabetic order
- demonstrates aspnet/Razor#225 does not affect HTML attributes
Fix #1618, Add asp- prefix to custom attributes of MVC tag helpers
- update XML docs to reflect new HTML / custom attribute separation
- update `Exception` messages to use new attribute names
- update MVC tag helper sample to use new custom attribute names
- add missing `<input/>` tag helper `throw`s test

nits:
- reword a few comments and messages for clarity and consistency
- use `<exception/>` sections to describe what's thrown
- add "Reviewers" comments about current throws
 - note `<a/>` and `<form/>` are slightly inconsistent with others: `throw`
   if unable to override specified `href` or `action` attributes; rest
   `throw` only if custom attributes are inconsistent e.g. `<input/>` with
   `asp-format` but no `asp-for`
- create test tag helpers after all property values are available
Add functional test of MVC tag helper sample
- ensure future changes don't break this sample