Finish abstracting native client

Firstly, don't initialize textsecure.nativclient unless the browser
supports it. The mimetype-check trick is hewn from nacl-common.js.

Secondly, nativeclient crypto functions will all automatically wait for
the module to load before sending messages, so we needn't register any
onload callbacks outside nativeclient.js. (Previously, if you wanted to
do crypto with native client, you would have to register a call back and
wait for the module to load.) Now that the native client crypto is
encapsulated behind a nice interface, it can handle all that
onload-callback jazz internally: if the module isn't loaded when you
call a nativeclient function, return a promise that waits for the load
callback, and eventually resolves with the result of the requested
command. This removes the need for textsecure.registerOnLoadCallback.

Finally, although native client has its quirks, it's significantly
faster than the alternative (emscripten compiled js), so this commit
also lets the crypto backend use native client opportunistically, if
it's available, falling back to js if not, which should make us
compatible with older versions of chrome and chromium.
Fixup curve25519 module

Rename methods on the curve25519 interface to be a bit more high level.
Cleanup emscripten wrapper class, wrap long lines and such. Also add a
grunt task alias for building the emscripten compiled curve
implementation.
Avoid breaking changes in qrcode library

Due to syntax errors in its bower.json.
https://github.com/davidshimjs/qrcodejs/pull/39
Update libphonenumber

```
bower install
cd components/libphonenumber-api
./build.sh
grunt
```
Closes #54