added option to allow renaming of metavars. Fixes #4.
changed default metavar names to their long forms: $c -> $count, $t -> $type, etc.
renamed ObjectId type to objectid (consistent lowercase with number/string).
fixed $prob calculation.
added Date type to data inference.
added date and null types, dates are also summarized by min and max.
Merge branch 'develop'

Conflicts:
	README.md