Problem: code for zclock_mono is slightly too complex

Solution: refactor functions that are used only once, to make the
code easier to reuse.
Problem: need a microsecond monotonic clock

This is for performance measurements where milliseconds don't cut it.

Solution: add zclock_usec() as alternative to zclock_mono().
Fixed error for zclock_usecs prototype
Removed labs area

Fixes #757

Note, if anyone wants this area for development the cleaner solution
is to create a separate zlabs project under the ZeroMQ organization.

Having this in CZMQ isn't worth the extra pain of carrying it around
in build files, documentation, etc.
Updated generated classes (zproto updates)
Problem: long extension for Makefile supplement is surprising

We don't use long extensions anywhere else and this is surprising and
clumsy IMO. I don't know if there's a standard for this.

Solution: use a short extension (.ams).
Problem: proxy switch code isn't minimal

Solution: simplify it a little more.
Problem: need a way to count zmallocs in a multithreaded app

Given that mallocs/frees are the most common cost in a high volume
app, it would be useful to get a rapid indicator of the total number
of allocations made (assume every alloc has a matching free and this
is checked with valgrind).

Solution: count mallocs in a process-global variable.

Note: done for GCC / Linux only in this instance, where we have the
necessary atomics.
Problem: need to be able to send unsigned integers via zsock_send

Solution: added 'u' picture type.

(Also minor space fix to zhash.c)
Merge pull request #755 from hintjens/master

Added microsecond clock for measuring time spans + more