Add a realloc ctx to bson_writer_new

Add a realloc_func_ctx to bson_writer_new and thread an additional
realloc_func_ctx on bson objects to allow for custom reallocs with a
handle.  This is to provide support for custom allocators without global
storage.  One that comes to mind is a pool allocator, where the context
object is a specific pool.
Add a couple more useful helpers
Merge pull request #49 from paulmelnikow/helpers

Add a couple more useful helpers
Merge pull request #51 from hanumantmk/realloc_func_ctx

Add a realloc ctx to bson_writer_new
bson: add bson_destroy_with_steal().

This allows, for non-read-only bson_t, to steal the underlying buffer.
In the case of inline buffers, a new buffer is allocated for the result
and returned for the convenience of the caller.

The caller must call bson_free() on the result when they are done with
the data.
bson: be a bit more defensive with stealing data.
bson: make bson_destroy_with_steal() equivalent to bson_destroy() if !steal
doc: add docs for bson_destroy_with_steal().