Replaced _compileSelector logic with non-eval'd code.

Implement $elemMatch.

(Commits squashed together by glasser, and some now-dead code removed.)
Some modifications to Ed-von-Schleck's selector compiler patch:

- Improve regex match against array.
- Improve matching of explicit deep nulls.
- Instead of doing a high-level "catch TypeError", do more explicit
  type checks.
- Clone the selector (since we're going to save a reference in our
  closure).
Rewrite selector compiler against with a "compile to specific function"
approach.
Rewrite sort compiler to avoid eval, too!
Merge branch 'selector-compiler-rewrite' into devel

This is a complete rewrite of the Minimongo selector and sort compiler to no
longer use eval, started by @Ed-von-Schleck and finished by @glasser. It makes
the code more readable and no longer requires all queryable types to be
serializable in an eval-able way; the latter will be useful once the in-process
EJSON work on the ddp-pre1 branch is ready.

(It also makes selector compilation faster, though selector evaluation appears
to be very slightly slower.)
Adjust accounts-twitter to use Twitter's id_str instead of id
Remove old XXX comment about $elemMatch needing tests.
Upgrade jquery-layout to 1.3.0RC. The jquery-latest site recommends it over 1.2.0 despite the RC designation. Fixes #556.
Don't drop quiescence state on initial connect.

We may have already run a stub before getting 'connected'! (Wordplay does this.)

(The same fix, without test, was already committed to ddp-pre1.)
Upgrade less to 1.3.3.

Should improve the error message in https://github.com/meteor/meteor/issues/203
(see https://github.com/cloudhead/less.js/issues/990) though it may not fix the
way that the error is handled.
Switch less back to fully synchronous, now that the NPM module supports it.

syncImport was added in 1.3.2 and we just upgraded from 1.3.1 to 1.3.3.

Otherwise error handling gets pretty fussy (eg, sometimes less throws from
within a callback).
Document how to prevent your less imports from being double-processed.
Merge branch 'dev-bundle-less-upgrade' into devel
First draft contributing file. Basically just copied from the wiki.

Once this is released to master, we should update the wiki to point to it.
tweaks
Choose an even more portable CSS property to test @imports.

Colors were showing up as rgb(a, b, c) in some browsers and as #aabbcc in
others.
Explicitly make {$regexp: "foo", $options: "garbage"} throw.

This check was explicitly in the old selector compiler but the rewrite dropped
it. The `new RegExp("foo", "garbage")` throws on most browsers but not on old
WebKit.
Added fix, workaround handlebars issue and added tests
Use regexp instead of isNaN for detecting integers.
Move tests from handlebars to templating, as they depend on templates. Also, extend to test a few more types.