Create and use versioned package dir by default.

* 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:

```julia
   _       _ _(_)_     |  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