Add a wtperf page to the documentation, how to simulate workloads.

Restructure the top-page a little, split apart admin and programming
sections.
Forgot to add the wtperf page.
Merge pull request #1147 from wiredtiger/wtperf-docs

Add a wtperf page to the documentation.
DEF_OPT_AS_INT isn't actually used anywhere, remove it for now.
Create a standalone program that outputs the wtperf documentation in
doxygen format, then compile & call it as part of running dist/s_docs.
Reference #1147.
Move CONFIG_OPT enum/structure into config_opt.h, move doxygen DEF_OPT
same structures/configuration as wtperf.  Reference #1147.
Remove extra reference to bulk-load page.  Rename tune_bulk_load for
consistency.
Fix references to new tune_bulk_load doc page.
Add a section to docs for async operations. #1139
The Linux ed command writes line numbers to stderr, redirect both
stdout and stderr to keep things quiet.
Add a new OS-layer function __wt_fsync_async to flush a file without
waiting for the results, call it from the Btree flush-leaves code so
pages start flushing while we're working the rest of the checkpoint.
Reference #1136.
FreeBSD doesn't have sync_file_range, add "unused" macros for the
__wt_fsync_async function's arguments.
Don't flush leaf pages asynchronously for now; reference #1152.
Clean up 3a8ed75, I was sloppy.
Merge branch 'develop' into async-doc
Review/edit the new asynchronous doc page, lots of random
cleanup/simplifications (at least, hopefully, they cleanup
and/or simplify).
Uninitialized variable, thanks, Jenkins!
Add a note about WT_ASYNC_OP handles.
Add WT_ASYNC_OP, WT_ASYNC_CALLBACK and WT_TXN_NOTIFY to PREDEFINE.
Merge pull request #1148 from wiredtiger/async-doc

Add a section to docs for async operations, closes #1139.