Performance improved for conversations switching

Now storing last N (N = 10 at the moment) conversations in DOM to be
able to show them really fast.
Improved statuses updates
Switch conversations performance improved

Improved performance: replaced watchers from templates with events
Conversation switch is now triggered by mouse-down, not click
Fixed search messages bugs

Fixed unread count in search - Closes #455
Fixed emoji spritesheet bug

Closes #454
Improved styles
created i18n-service and configuration in settings
added i18n support for ngPluralize

transparently localize ngPluralize directives by translating then value
of the "when"-attribute before ngPluralize compiles it
put current locale in consideration for various caches in filters/services

some local caches now need to be purged when the locale is changed on
runtime or else they'll still display strings in the wrong locale/language

Revert this commit once the locale loading is moved into init phase of
the page so it won't change during runtime anymore
integrated i18n in all modules

every string in the modules to be displayed should now be passed through
`_` first
integrated i18n into all partials

as in the previous commit every string to be displayed in the partials
should now bei passed through `_` first
created locale en-us for i18n

extracted all strings out of the partials and modules
added documentation/guide for the new i18n module
added german locales
increased width of .navbar-offline

localized strings may be longer and would wrap outerwise, hence added a
bit more room
allow the msgid to be passed as attribute instead of as html

without modification witching to key_ids the my-i18n directives would
something like this:

> <h5 my-i18n>Contacts</h5> or
> <h5><my-i18n>Contacts</my-i18n></h5>

but now can be written as

> <h5 my-i18n="im_contacts_header"></h5>
> <h5 my-i18n msgid="im_contacts_header"></h5>
> <h5 my-i18n>im_contacts_header</h5>

> <h5><my-i18n msgid="im_contacts_header"></my-i18></h5>
> <h5><my-i18n>im_contacts_header</my-i18></h5>