More convenience functions

Export colnames!()
Add vector() conversion function
Add subset() as an alias for sub()
Add a draft version of isless() for DataFrames
Add DataVec(Range1) constructor
Updated repl_show to include type info
More functionality; draft spec as a walkthrough

Added a new demo in demo/examples.jl
Added lots of colwise functions like colsums
Added comparison operators for DataFrames
Added dvzeros(Type, length) and dvones(Type, length)
Added dvfalses() and dvtrues()
Moved all operator definitions into one file.

Careful ordering of this file allows for proper definitions of NA
interactions with DataVecs and DataFrames.
Added the operators file
Added any() and all()
Added isnan() and isfinite()
Basic operator tests for spec preparation
Nearly finished operators tests; fixed broken operators
Cleaned up operator definitions
Added Pandas-style constructors; unique(DataVec)
No more ambiguity warnings.

Also renamed svd.jl to linalg.jl in preparation for DataMatrix type.
Fix lm.jl for the Distributions.jl change, and have map on
GroupedDataFrames return a Dict instead of a Vector{Any}.
Fixed eval-induced bugs;corrected def of :(.^)
Fixed NA^2.0 bug
Initial steps towards removing naRule's
Nearly working draft
Fixed constructor
Revised example of failure induced by load()
Run tests