Kill TBODY special case in DomRange

Background: When browsers parse HTML, they always insert a <tbody> around your <tr>s if you don't have one.  That is, `<table><tr>` becomes `<table><tbody><tr>`.  However, if you use the DOM API (appendChild et al.), this doesn't happen.  According to the latest specs and modern browsers, the TBODY element is not necessary in the DOM (though it is still always inserted when parsing HTML).  IE 6/7 disagrees, but that's history.  jQuery bends over backwards to insert TBODY elements dynamically, whenever you put a TR inside a TABLE.  Previously, we did too, but it's even harder for us.

This commit removes the code that special-cases tables in DomRange.  The code was never completely finished, and it will slow us down to have the extra essential complexity in DomRange.  Now, when you write `<table><tr>`, you'll get `<table><tr>` in the DOM tree.

It's important that we never optimize `<table><tr>...` into innerHTML, or else a template like `<table><tr><td>Foo</td></tr></table>` will get an inserted TBODY, while the same template with `{{foo}}` instead of `Foo` won't (because the table can't be optimized into static HTML).  A new test checks that we do this right.

The main downside of this change is that jQuery isn't as good for manipulating tables created by Meteor.  However, I think we can live with that.  jQuery made a particular choice there, and any problems that come up will be consistent across browsers and have workarounds (like inserting a <tbody> or not using a <table> tag).

Another downside concerns server-side rendering.  Rendering a template to a complete HTML string containing `<table><tr>` and then splatting that into the page will produce <tbody> tags that wouldn't be there if the page were rendered on the client.  We'll cross that bridge when we come to it.  In most ways (e.g. CSS rules), it shouldn't matter if there are <tbody> tags or not.
Convert relative urls to absolute url when merging CSS files
Add tests for the CSS url rewriting
Add `minifiers-tests.js` into package.js` (tests were not executed)
Handle the case where there are more than one url in the same value
Merge pull request #1986 from mquandalle/css-urls

Convert relative urls to absolute url when merging CSS files

Fixes issues introduced in `css-minification` branch in development mode and caused by concatenation in general.

When a stylesheet refers to an asset by a relative url, the relative url becomes invalid after concatenation if the stylesheet was originally located somewhere nested.
DDP Heartbeats

Add "ping" and "pong" messages to DDP.  This allows us to detect at
the DDP level when we've lost the connection.

Bump the DDP version to "pre2".  Preserve backwards compatibility by
not enabling pings if the negotiated DDP version is an earlier version.

Since receiving a ping indicates that the connection is alive, one
side of the connection doesn't have to send its own pings as long as
its receiving pings from the other side.  The ping interval defaults
to 30 seconds on the server and 35 seconds on the client, which means
that normally the pings go just one way (saving on bandwidth).

Increase the sockjs heartbeats from 25s to 45s, so they do not
normally fire.
Move the ddp-heartbeat test into tools/tests.
Move protocol out of heartbeat.js
Stop heartbeats on client side disconnect.

Use heartbeatInterval: 0 to disable heartbeats for testing.

No need to add `_internal` to the public connection API because we
can get to the internal session through the server.
Allow user to set heartbeat interval to 0 to disable sending pings.

Add test flag `_respondToPings` which allows tests to disable
responding to pings.
Add `ping` and `pong` to DDP spec.

Minimal writeup. Could stand to be fleshed out a bit. Maybe later.
Up sockjs heartbeat timeout (60s->100s) to match new sending interval (25s->45s)
Add debug print on timeout.

We have this now for sockjs timeouts, and it seems handy to have for DDP timeouts.
Comments.
Only print timeout message on the client.
Merge branch 'ddp-heartbeats' into devel
DDP spec: wrap heartbeat section
upgrade mongo to 1.4.0-rc9 from a fork of 1.3.19
checkpoint towards running multiple mongods

still need to make onExit blow it all up
checkpoint again