pick a non-conflicting module name when loading an extension prefix the module name by 'hgext_' so that it doesn't conflict with any core python module (for example bisect -> hgext_bisect) fix issue271

<item>
    <title>#tag|escape#</title>
    <link>#url#?cs=#node|short#</link>
    <description><![CDATA[#tag|strip|escape|addbreaks#]]></description>
    <pubDate>#date|rfc822date#</pubDate>
</item>