Make subscriptions handles reactively stop.

Although `handle.ready()` became false when the subscription was stopped, it didn't do so reactively, 
which led to some difficult to understand issues.
Added test case for reactive handle argument changing.
Explicitly test for sub arguments changing before ready.
Ensure the sub reactively goes unready when removed.

Subscriptions are removed from 3 points - handle.stop, nosub message, and _unsubscribeAll. We need to handle all three cases similarly.
Removed redundant checks for Deps' existence.
Subs’ readiness should not invalidate if they stop when they aren’t ready.
Fix for #2175 - Spacebars block comment cause unexpected errors

This fixes an edge case where a comment is followed immediately
by an {{else}} token.
Merge pull request #2180 from arbesfeld/comments-fix

Fix for #2175 - Spacebars block comment causes unexpected errors
Fix #2157 (issues with `=` in template args)
Add _nestInCurrentComputation flag to UI.render.

Internally, Blaze sometimes relies on `UI.render`'s autoruns nesting
within the current computation, but it's often the case that callers
don't want `UI.render`'s autoruns to be stopped when the outer
computation is invalidated. This change fixes a bug in {{#each}}: if
`addedAt` ran inside a `Deps.autorun`, and then the outer computation
was invalidated, the corresponding item would stop updating because the
autoruns set up by `UI.render` would be stopped.

Fixes #2156.
{{> UI.dynamic }} for easier dynamic template picking.

See #2123.
Use Meteor.absoluteUrl() to redirect in force-ssl
Drop collections after test.

Only on the consistent id test, but that one is run repeatedly.

Written using cleanup functions; we might want to make this part
of tinytest in future.
Switch the 'unreachable' url from http://asdf.asdf to http://0.0.0.0

Some ISPs / proxies serve a valid HTTP response on any DNS miss, but
we are trying to test an unreachable server.
Expose underscored animation hooks and add a test
Avi/David comments
Reshuffle packages to allow ui to depend on ui-dynamic-template.

Remove the spacebars-compiler -> spacebars and spacebars-compiler -> ui
dependencies. With those dependencies, ui cannot depend on
ui-dynamic-template, because ui-dynamic-template depends on templating,
whose build plugin depends on spacebars-compiler, who depends on
spacebars who depends on ui.

spacebars-compiler doesn't actually need to depend on ui or spacebars;
it only needs to depend on spacebars to add things to the `Spacebars`
namespace, which this commit does by factoring out the Spacebars export
into a separate `spacebars-common` package.
Change dynamic-tests.* file names
Add helper to check arguments to {{> UI.dynamic}}
Add some comments about UI.dynamic templates