1167 Guesser: use index scan queries

Modify the guesser sampling code in order to use index scan queries
instead of seq scan.
1167 use constant for IDS_COLUMN
1167 Move the sampling code to a TableSampler class
1167 Add unit tests for the TableSampler class
1167 Add a bit of documentation to TableSampler
1167 Better UI feedback and guessing time to the logger
Merge pull request #1186 from CartoDB/1167-guessing-no-seq-scan

1167 guessing no seq scan