Add new colormap for attachment sybol with sane fallback for existing colorschemes.

Squashed commit of the following:

commit 8dcf98df8e9104a5dc75539f64512cc91ed7eeba
Author: madhat2r <MaDhAt2r@dukefoo.com>
Date:   Wed Apr 23 08:48:29 2014 -0500

    Add new colormap for attachment sybol with sane fallback for existing
    colorschemes.

commit 78007ef68d55d79b5f8209cdc23a9644808a3962
Author: madhat2r <MaDhAt2r@dukefoo.com>
Date:   Tue Apr 22 18:54:14 2014 -0500

    Add new colormap for attachment symbol

commit 2a5b5d686271e0d21c3a9480cdfb5cc4ee54997e
Author: madhat2r <MaDhAt2r@dukefoo.com>
Date:   Tue Apr 22 15:43:43 2014 -0500

    Add optional colormap for attachement symbol
history: new :with_attachment
thread_view_mode: Kill a message (and next, prev etc..)
sup-sync: error msg on :update
Merge #300: sup-tweak-labels sync back unless specified otherwise

Squashed commit of the following:

commit ebfb3335070bba8e27551dcc4c232a4de2e98044
Author: Wael M. Nasreddine <wael.nasreddine@gmail.com>
Date:   Wed Apr 30 22:26:10 2014 -0700

    sup-tweak-labels: add a newline before the begin

commit f1d51d3961b0864c5f05f7b728cf5dd722a967eb
Author: Wael M. Nasreddine <wael.nasreddine@gmail.com>
Date:   Wed Apr 30 22:22:23 2014 -0700

    sup-tweak-labels: sync_back (honor the settings global and source) and provide an override

commit c43319a5fdfb725455beda51b7ec08799ecdda89
Author: Wael M. Nasreddine <wael.nasreddine@gmail.com>
Date:   Tue Apr 29 15:54:01 2014 -0700

    sup-tweak-labels: sync back if specified
sup.gemspec: Standarise the Gemspec
fix travis. The rake task to build a gem is now 'rake build'
Merge #303: Update gemspec and rakefile to use bundler
travis: drop 2.1.0 testing
Merge #301: thread_view_mode: Kill a message (and next, prev etc..)
history: bundler and threadkilling
prepare for 0.18.0
Merge branch 'develop'
release 0.18.0