Some minor cleanup

- It's not necessary to take an address when calling a method
- v, _ := m[k] is equivalent to v := m[k] when m is a map
- k, _ := range ... is equivalent to k := range ...
- Use for...range in a few places
Use driver.RowsAffected type
Use IndexByte to find string terminator

No performance gain since bytes.Index special cases this anyway, but it means
stringTerm can go away.
Remove stmt.nparams

It was always equal to len(st.paramTyps) and therefore redundant.
Adding test for interleaved statements in a transaction

See #81. Currently fails.
Move xact test to own file, since skip is go1.1+

This is less than ideal since the problem affects <1.1 as well,
but there doesn't seem to be a great way to include a failing
test otherwise, and time will paper over the grossness.
Merge pull request #116 from edsrzf/cleanup

Some small cleanup
Configure pq for Travis CI builds for interesting versions

 * 1.0.2: current stable Debian version
 * 1.0.3: most recent 1.0 version
 * 1.1: newest stable version
 * tip: keep an eye on what's coming up
Add Travis CI tag / link to README
add advisory lock