multi-ghc recipe, ghc-select script

`ghc-select` script for changing current version of `GHC`-compiler and related stuff

```bash
travis@oneiric32:~/ghc-multi$ ./ghc-select     
Usage:
 ./ghc-select                                    Show this usage information
 ./ghc-select list                               List available ghc versions
 ./ghc-select set <version>                      Set symbolic links to current version

travis@oneiric32:~/ghc-multi$ ./ghc-select list
7.0.4
7.4.2
7.6.3


travis@oneiric32:~/ghc-multi$ ./ghc-select set 7.10.2
Error: switch_to, no such version 7.10.2

travis@oneiric32:~/ghc-multi$ ./ghc-select set 7.4.2
`/usr/local/bin//ghc' -> `/home/travis/ghc-multi/bin/ghc-7.4.2'
`/usr/local/bin//ghc-pkg' -> `/home/travis/ghc-multi/bin/ghc-pkg-7.4.2'
`/usr/local/bin//haddock-ghc' -> `/home/travis/ghc-multi/bin/haddock-ghc-7.4.2'
`/usr/local/bin//ghci' -> `/home/travis/ghc-multi/bin/ghci-7.4.2'

travis@oneiric32:~/ghc-multi$ ghc --version         
The Glorious Glasgow Haskell Compilation System, version 7.4.2
```