IMPALA-724: Support infinite / nan values in text files

This patch allows the text scanner to read 'inf' or 'Infinity' from a
row and correctly translate it into floating-point infinity. It also
adds is_inf() and is_nan() builtins.

Finally, we change the text table writer to write Infinity and NaN for
compatibility with Hive.

In the future, we might consider adding nan / inf literals to our
grammar (postgres has this, see:
http://www.postgresql.org/docs/9.3/static/datatype-numeric.html).

Change-Id: I796f2852b3c6c3b72e9aae9dd5ad228d188a6ea3
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2393
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: jenkins