Freeze enforcing: (RIPD-399)

This is the rebased version of David's https://github.com/ripple/rippled/pull/473 to enforce freeze:

 * Set enforce date: September 15, 2014
 * Enforce in stand alone mode
 * Enforce at source
 * Enforce intermediary nodes
 * Enforce global freeze in get paths out
 * Enforce global freeze in create offer
 * Don't consider frozen links a path out
 * Handle in getBookPage
 * Enforce in new offer transactors
Is there an optimization from declaring `sle` near the top of the function and then assigning to it in the two different if conditions?  In terms of data locality it seems like we'd be better off making two different `sle`s inside the two different ifs.  But not a big deal.