Add a special "local" extension target that can call out to initialization functions within an application.

This will fail when attempting to run the `wt` utility, but it supports cases where extensions cannot be separate from application logic (e.g., custom collation routines).
Fix the metadata passed to WT_COLLATOR::customize
Add a page on writing WiredTiger loadable extensions, explain the special "local" usage.
Merge pull request #1174 from wiredtiger/mongo-collators

Fixes for MongoDB collators. Add support for loading extensions from application binary.