added cmake build to pion

I've implemented draft of cmake build for pion.

Could you please review? And accept or reject it.

Currently I've tested it on Linux(OpenSUSE 12.3 g++ 4.7.2) and Windows 7 (MSVC++ 2012 x64)
1. Linux - all unit tests pass
2 Windows - need some love (not all UT green - investigating)

ToDo:
- Add install target (NSIS, rpm)
- Fix all UT on Windows
- Add detection for ICU, python, iconv...

How to generate solution/build

1. Create out-of-source dir for target
2. Go to this folder
3. Run cmake <path to pion source root> [-G<generator you wish to use>] [-DBOOST_ROOT=<path to boost root>] [-DOPENSSL_ROOT_DIR=<openssl root dir>] [-DLOG4CPLUS_ROOT=<path to log4cplus root dir>] [-DZLIB_ROOT=<path to zlib>]
4. See if generation successful and run your build tool