Aura themes refactoring

switch from aura:attribute to aura:var
add component bundle themes
switch to global var references and overrides

@bug W-2024194@
@bug W-2024187@
@bug W-1621197@
Merge branch 'themes-two' of git.soma.salesforce.com:nmcwilliams/aura into bob/master-pending/pr-53-aura-themes
Merging origin/bob/master-pending/pr-53-aura-themes into master