{{linkTo}} bound contexts, loading class

Merge of work between @drogus and @machty.

Replaces and Closes #2181
Replaces and Closes #2252

{{linkTo}} contexts are now bound. e.g.:

    {{linkTo 'foo' pathToObj}}

will generate a linkTo that updates its
href if the resolved value of `pathToObj`
changes.

Also, if ANY of the linkTo params yield
null/undefined, including the first route name
param (if it is unquoted), href will be '#',
a new `loadingClass` class will be applied to
the linkTo element, and clicking/invoking
the loading linkTo will no-op and produce
an Ember.Logger.warn message that the linkTo
isn't loaded yet.

Note: due to overlap in loading logic and the recently
added stringified linkTo route behavior, the loading
logic is hidden behind the ENV.HELPER_PARAM_LOOKUPS
flag for now.