Update .gitignore
Implemented three extraction methods

[, [[, $ extraction methods

[[<-, $<- replacement methods
Added MADlib version check

Only newer versions of MADlib are supported.

So if the user tries to run MADlib wrapper function, he will get an
error message complaining that a newer version of MADlib is needed.
Implemented Arith and Compare operators

for db.obj and numeric etc
Fixed several small bugs
Added the support for categorial columns
Added summary function for db.obj
Fixed all obvious bugs
Increased the version to 0.0.2
Fixed more bugs
Better way to handle distributed.by & More efficient way to find
distinct values for multiple columns

Scan the table only once to find all distinct values
Added all needed operators

More importantly, improved the SQL query generation
Added the replacement method of [
Added merge method
Added key functions

Make it easier to change the key
Fixed bugs
Added 'sort' and 'by'

Also some aggregate functions
Fixed lots of bugs and increase the version

to 0.0.3
Added the ability to deal with column names with quote
Minor change