value: add bson_value_t for convenience boxed type. [ABI Break]

This adds a new type, bson_value_t for retrieving boxed types from
a bson_iter_t. It will make binding from other languages a bit simpler.
Also, I intend to make bson_visitor_t much simpler using this type as
we can consolidate callbacks into just a couple of them.
value: add bson_append_value()
build: add new symbols to ABI files.
B-0 Fixed warning compiling in Visual Studio
B-0 Fixed compilation warnings and errors in Visual Studio
Merge pull request #34 from Convey-Compliance/fixed_to_version_0.6.5

Fixed warnings compiling in Visual Studio
build: fix some more warnings.
B-0 Fixed some warnings when compiling in 64 bits
B-0 Finished removing warnings compiling in 64 bits
build: enable maintainer flags when doing distcheck.
build: drop thin archives, not useful enough and breaks CentOS 5.
B-0 Fixed small warning missing typecast.
VS doesn't like assigning a regular number to an enum
Merge pull request #38 from Convey-Compliance/fix_small_msvc_warning

B-0 Fixed small warning missing typecast.
Merge pull request #35 from Convey-Compliance/fix_warnings_compiling_64bits

Fix warnings compiling 64bits
build: add bson-value.{c,h} to cmake
build: add missing file to cmake.
cbson: cleanup some warnings.
iter: add BSON_ITER_IS_KEY macro
Fix documentation

bson_iter_date_time returns the unix timestamp in miliseconds.
Merge pull request #41 from mcuadros/patch-1

Fix documentation of bson_iter_date_time
macros: add BSON_GNUC_DEPRECATED and BSON_GNUC_DEPRECATED_FOR