Bump to 1.2.0 -- Adding snapshots to recon_alloc

This commit does a few minor fixes to @garazdawi's snapshot addition by
adding a few export clauses, adding or changing the documentation a bit,
and bumping the version number to 1.2.0
Change average_sizes to average_block_sizes

When figuring out what sizes to have in your lmbcs, smbcs or sbct it
is the average block size that is interesting. This is because we
usually want to modify the limits to fit as many blocks as we want in
one carrier. The average carrier size is easy enough to figure out by
just looking at the configurations of the allocators.

This is a backwards incompatible change.
Update docs for sbcs_to_mbcs
Rewrite of recon_alloc:memory

Added a two new options; allocated_types and allocated_instances
that can be used to figure out which allocator types/instances most
data lies in.

Added a new parameter to specify whether you want the current or max
value. This works like sbcs_to_mbcs and you want it for the same reasons.

Change memory(used,_) to use the total block size rather than
erlang:memory(total). This number should be the same, but you can also
get the max value.

These changes are backwards incompatible with the old API
Added current and max options to sbcs_to_mbcs

This is a backwards incompatible change