Ubuntu build fixes and instructions

This pull request makes Impala build on Ubuntu 12.04. It basically contains these changes:

* Fix the linker settings to use `--no-as-needed` for executables (`--as-needed` is default on current Ubuntu distributions).
* Fix boost linking setup.
* Add an explicit copy assignment operator to `FragmentExecParams` to work around a bug with Boost 1.4x and newer compilers where Boost generates a copy assignment operator that takes a non-const argument, but newer compilers/STL require a const argument.
* Add support for versioned llvm binaries (e.g. `llvm-config-3.0`) as llvm ubuntu/debian packages don't (currently) use the alternatives system for these.
* Fix the maven repository settings (#8).