Update contribution docs concerning cross-browser tests

[RichardLitt]

There are more tests that run on the Travis server than are listed in the unit testing section in the Contribute docs. grunt test:unit doesn't include the cross-browser tests, and there are no instructions on how to run all of the tests that are run on Travis. Also, some of the tests listed as possible in the docs don't work on a clean, updated fork - note Opera not being registered, below.

There may be an easy fix to this; for instance, suggesting that the users set up a Saucelabs account, or use Travis's, or that they preinstall a few extra modules. Either way, it should be in the docs.

~/Github/angular.js $ grunt test:unit --browsers Opera,Firefox
Running "tests:jqlite" (tests) task
INFO [karma]: Karma v0.12.9 server started at http://localhost:9876/
WARN [launcher]: Can not load "Opera", it is not registered!
  Perhaps you are missing some plugin?
INFO [launcher]: Starting browser Firefox
INFO [Firefox 28.0.0 (Mac OS X 10.9)]: Connected on socket 0W2CN5mA2DzUtzaP0NLB with id 76848034
Firefox 28.0.0 (Mac OS X 10.9): Executed 3000 of 3000 SUCCESS (8.791 secs / 8.511 secs)

Running "tests:jquery" (tests) task
INFO [karma]: Karma v0.12.9 server started at http://localhost:9876/
WARN [launcher]: Can not load "Opera", it is not registered!
  Perhaps you are missing some plugin?
INFO [launcher]: Starting browser Firefox
INFO [Firefox 28.0.0 (Mac OS X 10.9)]: Connected on socket 9EMnouvPQ0um3jUg0QBr with id 68862994
Firefox 28.0.0 (Mac OS X 10.9): Executed 3009 of 3009 SUCCESS (9.463 secs / 9.21 secs)

Running "tests:modules" (tests) task
INFO [karma]: Karma v0.12.9 server started at http://localhost:9876/
WARN [launcher]: Can not load "Opera", it is not registered!
  Perhaps you are missing some plugin?
INFO [launcher]: Starting browser Firefox
INFO [Firefox 28.0.0 (Mac OS X 10.9)]: Connected on socket wM6Q1mnnJFJG2f0E0S51 with id 33078552
Firefox 28.0.0 (Mac OS X 10.9): Executed 385 of 385 SUCCESS (1.134 secs / 1.086 secs)

Done, without errors.

Version: v1.3.0-beta.7
Reproducible: Always
OS: OSX 10.9.2
Steps to reproduce:

• Fork project, get master
• Run grunt test:unit --browsers Opera,Firefox

[btford]

Thanks @RichardLitt!