forward-declare struct iovec

avoids warnings of the form:

warning: 'struct iovec' declared inside parameter list
warning: its scope is only this definition or declaration, which is probably not what you want

when building downstream projects
Merge pull request #388 from minrk/forward-iovec

declare struct iovec
Fix a bug in trie implementation

When removing keys for a specified value, make sure we drop
empty node table. Failing to do this can lead to asserion failure.

Refs: http://lists.zeromq.org/pipermail/zeromq-dev/2012-June/017589.html
Merge pull request #389 from hurtonm/master

Fix a bug in trie implementation
Small code simplification
Merge pull request #390 from hurtonm/master

Small code simplification
Let docs for zmq_ctx_set_monitor() respect a 80 char wide column width
Merge pull request #391 from methodmissing/monitor-manual-format

Let docs for zmq_ctx_set_monitor() respect a 80 char wide column width
mtrie: Do not throw exception on allocation failure
mtrie: Code simplification
Merge pull request #392 from hurtonm/master

mtrie clenaup
Making Android&pyzmq friendly version with avoid-version
Merge pull request #393 from cyrilh/master

Making Android&pyzmq friendly version with -avoid-version
Fixed documentation of HWM on ZMQ_PULL
Merge pull request #394 from hintjens/master

Fixed HWM doc on ZMQ_PULL
Fixed issue #393, compilation warnings
Merge pull request #395 from hintjens/master

Fixed issue #393 - Android build error
Reverted fix for #393, was already fixed
Rewrite process_command's loop
Merge pull request #396 from hintjens/master

Reverting fix for #393