Remove unnecessary crossDomain flag on xhr

This is only needed to force the behavior of a cross-domain request
when requesting to the same domain.
Fix logic and simplify cilantro.isSupported

Previously the version passed set bounded the minimum version supported
which was wrong. This function has been changed to require a version to
be passed. The max version defaults to Cilantro's unless the API's version
is less than what Cilantro supports. For example, the following:

    cilantro.isSupported('2.1.2')

returns true if the current endpoint's version is at least 2.1.2. The
minSerranoVersion and maxSerranoVersion variables are simply used to
notify when Cilantro does not support (less than minimum) or has not been
tested (greater than maximum) the current version of Serrano.
Update maxSerranoVersion to 2.1.3
Add json2.js for IE 7 support

This does a conditional check to ensure the resource does not load for
browsers that support JSON natively.
Update router to empty the contents of main on the first render