FromClause.GetString() shouldn't assume 2 series in the merge

Fix #1047
Make sure the query string has valid regex and add a test
add some docs
Rename MergeFun to MergeRegex
Support merge with a list of series names
add a test to make sure merge cannot be used with multiple regexes
Merge pull request #1047 from influxdb/fix-1047

Allow merge() to work with one series and to take a list of time series