Standardize Variable substitution on EL syntax

When the spec adopted data pipelining in the 0.9 version of the spec, we added support for an expression language syntax loosely based on JSP EL. This also included overlapping support for features covered by the existing variable substitution syntax, such as message bundle syntax.

The older variable substitution syntax uses the format

 __<TYPE><key>__

whereas the EL syntax uses

${Key.Property}

and a number of reserved keys.

The EL syntax is more powerful, flexible, and easier to type. I'd like to see the spec deprecate the older and more limited double-underscore syntax for variables and standardize on the EL syntax. Also, the parts of the spec that deal with EL are scattered, so the specs should consolidate most of them into the Core Gadget spec, leaving the remaining template-specific sections in the templating spec.

Older variable substitution section

http://opensocial-resources.googlecode.com/svn/spec/2.0/Core-Gadget.xml#VariableSubstitution

Newer EL syntax sections

(expressions in templates)http://opensocial-resources.googlecode.com/svn/spec/2.0/OpenSocial-Templating.xml#rfc.section.7
(EL access to meta-data from data pipeline tags)http://opensocial-resources.googlecode.com/svn/spec/2.0/OpenSocial-Templating.xml#rfc.section.8

(Special EL variables in templates)http://opensocial-resources.googlecode.com/svn/spec/2.0/OpenSocial-Templating.xml#rfc.section.9

(Dynamic Parameters in Data Pipeline tags and ViewParams)http://opensocial-resources.googlecode.com/svn/spec/2.0/Core-Gadget.xml#rfc.section.13.8

(Localization and $

Unknown macro: {Msg}

variable)http://opensocial-resources.googlecode.com/svn/spec/2.0/OpenSocial-Templating.xml#rfc.section.16