Add ability to pull down Thunderbird extensions

Extensions should be put in netlogon/domain_information/thunderbird,
and must have the extension '.xpi'.
Remove Thunderbird LDAP directory settings

Contacts are now handled through SOGo
Add ability to pull down architecture-specific Thunderbird extensions

Global extensions should be in
netlogon/domain_information/thunderbird/extensions, while Linux-specific
extensions go in netlogon/domain_information/thunderbird/extensions/linux-ARCH.
Architecture name can be found using 'uname -i'. For example, Linux on x86_64
would use the folder extensions/linux-x86_64.
Fix errors in Thunderbird extension code; remove need for temporary directory
Make unzip output quiet, fix error removing files
Fix extension ID extraction using xmlstarlet to support attribute em:id
Add mail reader to application menu, and remove some excludes
Fix nested symlink error in Thunderbird extension installation
Convert all '[' tests to '[[', closes #25
Remove quotes around variables in '[[' tests
Requote right side of '==' in '[[' to prevent potential error

See: http://mywiki.wooledge.org/BashPitfalls#if_.5B.5B_.24foo_.3D_.24bar_.5D.5D_.28depending_on_intent.29
Add copyright notice to all scripts, closes #24, update copyright date
Change to match LDAP structure change in server version

See commit: 7bd8401b2d6f887cbf762b42f8e41cccab7ca004
Merge branch 'sogo'
Merge branch 'master' into install-karoshi
Add xmlstarlet to install-list