CCBC-375: Add doxygen API docs. Remove man3 manpages

This comprises the new API documentation system. The header layout has
changed to make the API more readable and easily documented.

Rather than having things organized by "Structure", most of the public
API has now been placed inside couchbase.h and functions/structures
organized according to their logical category rather than their datatype
(for example, all APIs pertaining to lcb_get() are consolidated in a
single place).

The documentation layout is controlled by a `Doxyfile` in the root
source directory which at this moment is slightly modified to accomodate
the current source layout. It is possible to generate man pages from
Doxygen, but our goal is to make an easily browsable HTML-based
documentation.

This change also performs some minor API cosmetic enhancements:

- Functions to be deprecated have been decorated as such. Specifically
  the list of functions to be deprecated are:

  * lcb_set_timeout (and all sorts of function-type timeout accessors.
    (lcb_cntl() should be used for this))
  * lcb_behavior_* functions (again, this should be done via lcb_cntl)
  * lcb_timer_* functions. These should have never been public API in
    the first place
  * lcb_verify_struct_size(), lcb_verify_compiler_setup(). The command
    structures are to be migrated to a newer "V3" API. The current
    command API will go away when we hit a 3.0 version.

- Nested structures have been exposed for easier documentation and
  access, thus they are no longer anonymous structures

- C++ "Wrapper" functions are declared as `inline` within couchbase.h
  and are "defined" in cxxwrap.h which is included when in C++ mode.
  These "wrappers" should also be considered deprecated, but too much
  (internal) code uses it right now and I wanted to make it minimal

Change-Id: Ib3b432d998ffb23a309402d01533ecd5321e693d
Reviewed-on: http://review.couchbase.org/36077
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>