Create and use versioned package dir by default.

[kmsquire]

• Change pkgroot => _pkgroot, to discourage use outside of pkg/dir.jl
• Add versioned package dir information to NEWS.md

Except for the location of the Git module, this should be identical to the changes in v0.3 (except all in one commit).

Tested with version 2, and it does indeed create and use a versioned package dir:

   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "help()" to list help topics
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.2.0+23 (2014-02-09 01:46 UTC)
 _/ |\__'_|_|_|\__'_|  |  kms/versioned_pkg_dir_julia0.2/647460a* (fork: 23 commits, 84 days)
|__/                   |  x86_64-linux-gnu

julia> Pkg.init()
INFO: Initializing package repository /home/kmsquire/.julia/v0.2
INFO: Cloning METADATA from git://github.com/JuliaLang/METADATA.jl

julia> Pkg.add("DataFrames")
INFO: Cloning cache of Blocks from git://github.com/tanmaykm/Blocks.jl.git
INFO: Cloning cache of DataArrays from git://github.com/JuliaStats/DataArrays.jl.git
INFO: Cloning cache of DataFrames from git://github.com/JuliaStats/DataFrames.jl.git
INFO: Cloning cache of GZip from git://github.com/kmsquire/GZip.jl.git
INFO: Cloning cache of SortingAlgorithms from git://github.com/JuliaLang/SortingAlgorithms.jl.git
INFO: Cloning cache of Stats from git://github.com/JuliaStats/Stats.jl.git
INFO: Cloning cache of StatsBase from git://github.com/JuliaStats/StatsBase.jl.git
INFO: Installing Blocks v0.0.1
INFO: Installing DataArrays v0.0.2
INFO: Installing DataFrames v0.4.2
INFO: Installing GZip v0.2.10
INFO: Installing SortingAlgorithms v0.0.1
INFO: Installing Stats v0.1.0
INFO: Installing StatsBase v0.2.10
INFO: REQUIRE updated.

julia> ;ls ~/.julia
v0.2

julia> ;ls ~/.julia/v0.2
Blocks  DataArrays  DataFrames  GZip  METADATA  REQUIRE  SortingAlgorithms  Stats  StatsBase

Note that we still could use migration functionality (#4876).

cc: @StefanKarpinski