RFC: Added ref type parameter, compact to PooledDataFrame

[kmsquire]

[Copied from https://github.com//pull/227; see there for additional comments/discussion. That pull was from my repository, and this one is from the DataFrames.jl repo, which makes it easier for others to test.]

• Allows the user to specify the reference type for PooledDataArrays
  (default is still Uint16)
• Added compact to take an existing PooledDataArray and change the
  reference type to the smallest size possible. Returns the same
  PooledDataArray if nothing has changed, and a new one if the size
  is reduced

The current PooledDataArray tests pass.

Example (from #241 (comment))

julia> using DataFrames

julia> a = PooledDataArray([1,1,2,2])
4-element Int64 PooledDataArray:
 1
 1
 2
 2

julia> a.refs
4-element Uint16 Array:
 0x0001
 0x0001
 0x0002
 0x0002

julia> b = compact(a)
4-element Int64 PooledDataArray:
 1
 1
 2
 2

julia> b.refs
4-element Uint8 Array:
 0x01
 0x01
 0x02
 0x02

julia> c = PooledDataArray([1,1,2,2], Uint8)
4-element Int64 PooledDataArray:
 1
 1
 2
 2

julia> c.refs
4-element Uint8 Array:
 0x01
 0x01
 0x02
 0x02

julia> c = PooledDataArray([1,1,2,2], Int8)
4-element Int64 PooledDataArray:
 1
 1
 2
 2

julia> c.refs
4-element Int8 Array:
 1
 1
 2
 2

[kmsquire]

This was already applied, but maybe got accidentally pulled along in the move from Harlan's repository. Closing...