Bug 906316 - Don't download the xulrunner SDK each time we change a branch if their configuration are different r=yurenju,asuth,ochameau

This also shows a message when the archive file seems to be corrupted, advising
to run "make really-clean". Also changed "make really-clean" to remove all
xulrunner related files.

We now uncompress the archive directly in a specific directory that should
change when we change the URL.

We use a dot file inside the xulrunner SDK directory to keep the URL used to
download that SDK.

Now we can also define XULRUNNER_DIRECTORY from the command-line. If not
defined, it will use the default xulrunner-sdk-26/xulrunner-sdk.
We can also define XULRUNNER_BASE_DIRECTORY which is the place where the
XULRunner SDK will get uncompressed.

We export the new XULRUNNER_DIRECTORY variable along with the
existing various xulrunner program paths. These paths have been made absolute
using make's `abspath` operation.

The patch also removes the ./ prefixes to those paths as they're likely useless
and prevent from using an absolute path when defining XULRUNNER_DIRECTORY from
the command line.

This patch also remove the email's build_stage directory when running "make
clean".

This _will_ make all devs redownload XULRunner, sorry.

This make the build fail if submakes fail

Also add documentation at the top of the Makefile.
Merge pull request #12942 from flodolo/bug928347

Bug 928347 - Add missing strings in settings.en-US.properties, r=evelyn
Merge pull request #12857 from pivanov/bug-917837

Bug 917837 - [Music] Use Full Line Highlights in Album View
Merge pull request #12689 from gasolin/issue-917359

Bug 917359 - Remember my choice is exposed for the getUserMedia dialog, ..., r=alive
Bug 927768 - Fix incorrect generated CSS transform value in card view
Bug 918998 - Hide email info in notification bar when there is only one configured account. Show email info when there are one more accounts.
Merge pull request #12959 from timdream/bug927768

Bug 927768 - Fix incorrect generated CSS transform value in card view, r=timdream
Bug 924244 - [B2G][Contacts] After exporting a contact the UI still displays Export in the top right
Bug 913385 - Add Hardware tests mode r=evelyn

    This patch organize old tests into API and UI parts, and add new Hardware tests.
    Also, there are some minor fixes on existing tests.
    HW tests mode provide the following tests:

    Audio loop
    Battery
    Bluetooth
    Camera
    Headphone
    LCD
    Physical keys
    Ringer
    SDCard
    Sensors
    Touch screen
    Vibrator
Merge pull request #12139 from arasbm/bug-909373-openaudio-activity-save-robustness

bug-909373-[Music] open activity saving feature robustness, r=@dominickuo
Merge pull request #12770 from jmcanterafonseca/fix_924244

Bug 924244 - [B2G][Contacts] After exporting a contact the UI still disp...
Merge pull request #12951 from wanderview/uitest-symlink-b2g

Bug 928631: support sym linked b2g in gaia-ui-test
Bug 928801 - [settings] Should use the term 'EVDO', 'CDMA' for CDMA network modes
Bug 912028 - Background is visible on keyboard pop in
Merge pull request #12499 from comoyo/keyboard_slide_up

Bug 912028 - Background is visible on keyboard pop in. r=RudyL
Revert "Bug 927817 - test_clock_turn_on_off_alarm.py is getting a StaleElementException against desktopb2g"

This reverts commit f846296e7b9222bf874409245e36102f1e862b77.
Revert "Bug 923448 - In test_clock_turn_on_off_alarm.py remove the sleep and add a wait_for_checkbox_state_to_change"

This reverts commit da8326f967be8e30052063c76685d1792868e492.
Bug 928169 - [B2G][Homescreen][Dock] Installing too many apps places icons behind dock UI
Merge pull request #12944 from wanderview/ftu-mock-icc-2

Bug 928069: Fix more ftu tests to use correct mock_icc_helper. r=fcampo
Bug 923574 - Ensure manifest size is valid

When installing an application with an invalid/undefined size, the new
WebIDL is not happy about this and the notification stays forever. We
fix this by checking that the size is effectively defined before setting
the max value of the notification.