WIP: add support for integer and NA values

[dernst]

• adds support for integers (INTEGER and SMALLINT are read as integers)
• adds support for NA values for integers and real values: NA values are writing as NULL and NULL is read as NA (for real values all NaN values are treated as NA)

I would probably like to avoid hard-coding the java.sql.Types values in R code. Perhaps we could query these constants in JDBC() when it's first used.

Let me know if you're interested in such changes. We're using these in production currently which seems to work well so far.

[dernst]

If #8 is going to be merged I'll edit my changes to include it.