This fixes Issue #42 and puts in at least one regression test for 64-bit
integer values, which work in node.js up to about 2^54 and down to
-2^54. May as well make a best effort to get them right.  This issue is
due to a regression introduced in commits  6b4c80c and 0b4f361, which
taken together always encode larger integers as doubles, even when
they shouldn't be.
A few more regression tests can't hurt...
Merge pull request #43 from maxtaco/issue_42

Since this could change packed/unpacked results, I am going to minor version bump the package once a get a few more things into master.