Update README.md
Merge pull request #202 from catawbasam/patch-1

Update README.md
Updated methods for manipulating levels in PDAs (#203).
More PDV/levels tweaks.
Merge branch 'master' of git://github.com/HarlanH/DataFrames.jl
Renamed reorder! to reorder and levels! to set_levels, and changed
the PooledDataArray to not sort the pool if the pool is specified (#203).
Add set_levels! (#203).
permute!!() is not exported by Base
Merge pull request #204 from kmsquire/patch-1

permute!!() is not exported by Base
Add capability to merge on multiple columns (#17).
Merge branch 'master' of git://github.com/HarlanH/DataFrames.jl
Lots of smallish changes to tweak functions, especially to make them
more abstract.
Switch from extras/gzip.jl to GZip package.
Update all uses of `expr()` and `Expr()` related to Base changes to
fix #207.
Merge branch 'master' of git://github.com/HarlanH/DataFrames.jl
Updates to adapt to Julia changes (immutable types and removal of
function composition).
Update REQUIRE for a dependency to Julia 0.2.
Another update for a dependency to Julia 0.2.
Switch to get/set methods
Fix ambiguous ref definition