API Disable discontinued Google Spellcheck in TinyMCE

Replaced by browser-based spellchecking if available (Chrome, Firefox),
with instructions on how to use PSpell as an alternative.
Validate 'archiveDate' user data in Versioned

Not a security issue as such, since the user input is sanitized
before being used in Versioned->augmentSQL(). But it shouldn't
reach the session state either, since that's commonly assumed
to be sanitized data, and it leaves unnecessary room for error.

strtotime() has fairly loose validation rules around dates,
but its a good "first line of defence".
FIX: Undefined variable in TreeDropdownField.php

I'm guessing this was just a simple typo. Checked this by searching on SiteTree, behaves as expected after this change.
Merge pull request #2476 from kinglozzer/treedropdownfield-undefined-var

FIX: Undefined variable in TreeDropdownField.php
BUG Sort column order maintained correctly when using expressions in SQLQuery and DataQuery
BUG Fixed cross-platform issues with test cases and file utilities
BUG Issue with login form failing to login in certain situations. Fixes issue #2424
Merge pull request #2480 from tractorcow/pulls/3.1-dologin-security-fix

BUG Issue with login form failing to login in certain situations. Fixes ...
BUG Issue with login form failing to login in certain situations. Fixes issue #2424
FIX: Make CreditCardField use template
Merge pull request #2468 from nedmas/make-creditcardfield-use-template

FIX: Make CreditCardField use template
BUG Fixing installer not checking display_errors correctly.

Fixes issue #2479. Installer sets display_errors on, but it checks
the changed value and not the original one set in php.ini.
Merge pull request #2478 from tractorcow/pulls/3.1-platform-compatibility

BUG 3.1 Fixed cross-platform issues with test cases and file utilities
Merge pull request #2477 from tractorcow/pulls/3.1-sqlquery-sort-order

BUG Sort column order maintained correctly when using expressions
Merge pull request #2328 from chillu/pulls/versioned-archivemode-validation

Validate 'archiveDate' user data in Versioned
Merge pull request #2288 from chillu/pulls/browser-spellcheck

API Disable discontinued Google Spellcheck in TinyMCE (#2213)
Merge pull request #2487 from halkyon/installer_error_check_30

BUG Fixing installer not checking display_errors correctly.
BUG Fixed cross-platform issues with test cases and file utilities
Updating unit and integration testing documentation.

Updating introduction summary, and fixing broken links.
Proof-of-concept of running Firefox Behat in Travis-CI