Implement a new element in AstXML for AMI actions documentation.

Implement a new element in AstXML for AMI actions documentation.

A new xml element was created to manage the AMI actions documentation,
using AstXML.
To register a manager action using XML documentation it is now possible
using ast_manager_register_xml().
The CLI command 'manager show command' can be used to show the parsed
documentation.

Example manager xml documentation:
<manager name="ami action name" language="en_US">
    <synopsis>
        AMI action synopsis.
    </synopsis>
    <syntax>
        <xi:include xpointer="xpointer(...)" /> <-- for ActionID
        <parameter name="header1" required="true">
	    <para>Description</para>
	</parameter>
	...
    </syntax>
    <description>
        <para>AMI action description</para>
    </description>
    <see-also>
    	...
    </see-also>
</manager>

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@196308 65c4cc65-6c06-0410-ace0-fbb531ad65f3