FIX: destroy amqp_envelope_t in consumer example

Make sure to destroy the envelope object in the amqp_consumer.c example
CMake: correct generation of librabbitmq.pc

Fix CMake code that substitutes librabbitmq.pc to be correct.
Removing distro-specific packaging components

It doesn't seem possible to write a distribution-agnostic .spec or .deb
build files that won't get changed by the distribution anyway. Instead
I'm going to let the downstream consumers create their own packaging
Non-exclusive queue declaration in amqp-consume.

Declare non-exclusive queue by default, which allows task queue
round-robin publishing when binding to a "direct" typed exchange with
a routing key matching the publisher's one and the queue name.
The "-d" option is replaced with a "-x" option to explicitly declare
an exclusive queue if needed.
amqp_socket_close() should not be a public fn
FIX: use correct buf/len when re-starting send()

Fix a bug that manifested itself by the broker disconnecting the client
with an invalid_frame_end_marker error. The cause was when send()
returned less than the size of the buffer to be sent we weren't using
the correct parameters when restarting the send call.
Prep for v0.4.1 release
Bumping version due to v0.4.1 release
Add amqp_basic_nack() API function
* tools/CMakeLists.txt: Fix missing POPT_INCLUDE_DIRS variable.
.gitignore: ignore editor swap files

Some source code editors create swap files in the same directory as the
file being edited. These should be ignored by git.
Fixed the library path for the x86_64 architecture
Documentation fixes

* amqp.h: Fix link to amqp_destroy_envelope()
* amqp_ssl_socket.h: Fix typo in parameter name
* amqp_tcp_socket.h: Use correct parameter name
* amqp.h: Typo in amqp_basic_ack() documentation
amqp.h: delivery mode constants

RabbitMQ defines two delivery modes, persistent and non-persistent, see
http://www.rabbitmq.com/amqp-0-9-1-reference.html#class.basic for more
information. This commit adds enumeration constants for these delivery
modes.
Switched to CMAKE_CURRENT_.. variants for source and binary dirs to enable nested builds.
Moved mutex initialization inside ENABLE_THREAD_SAFETY guards.
Right unexpected frames requeue in amqp_consumer.c
Use CMAKE_INSTALL_* variables provided by GNUInstallDirs.
Add GNUInstallDirs.cmake module

Adding GNUInstallDirs.cmake module for older versions of CMake that do not have
that module
Fix function amqp_get_monotonic_timestamp return zero

On one of my PC (Intel Core2 Duo Processor E7500 2.93 GHz),
QueryPerformanceFrequency return 2933330000.
This value  is greater than AMQP_NS_PER_S (1000000000).