check_for_missing_species() is already called in check_for_align_problems()
Tidying-up of content_sub_slice() arguments

content_sub_slice() does not deal with warnings any more (it was only
appending them to the HTML anyway). It is now up to the caller (in two
cases: content()) to create the HTML and add it to the page.
It was actually almost the case in Variation/Compara_Alignments.pm
Refactoring of Compara WGA warnings

show_warnings() is now in Shared, rather than in Compara_Alignments.
All messages now have a severity level (error, warning, or info) which is
passed as the first argument to $self->_info_panel()

Missing species now only trigger an info panel, not a warning.

check_for_align_in_database() was called first with the wrong args
($align_param which is a composite string, but the parameter should rather
be a dbID), and a second time within check_for_align_problems(). Only the
latter is now used, and now reports whether there were any (fatal) errors.
Multiple alignments also have their tree display

The tree display is adapted from the display used for Gene Trees and CAFE
Trees. The code fetches the GenomicAlignTree, which also inherits from
NestedSet, and feeds it into the widget.
Merge pull request #73 from muffato/genomic_align_tree_display_2

Genomic align tree display