Use get for all Dust references

dot-notation references will now use get, and getPath is deprecated.

Also in this commit:

* Update to version 2.2.0
* When get returns a function, add an `isFunction` attribute to the returned function and set it to true. This fixes the bug that was causing dustjs-helpers tests to fail.
Merge branch 'master' of https://github.com/linkedin/dustjs into getPath2get

Conflicts:
	lib/dust.js
Add tests from #357
Improve the public API for Context.prototype.get

The public API (to be used in helpers) now supports getting in the following ways:

'key'
'.key'                   // Only searches in the current context
'path.to.key'
'.path.to.key'           // Only searches starting in the current context
['key']
['path', 'to', 'key']
Improve efficiency of getPath fallback

Go straight from getPath to _get

Also, update documentation for _get
Update dist files
remove back compat from _get, because it's not needed anymore
Merge pull request #360 from smfoote/getPath2get

Use get for all Dust references