Support for boost [>]= 1.53

Hi, I wanted to try out licppa but found out it was not building due to some changes in context library and the addition of new coroutine library in boost 1.53.
This commit has fixed everything for me, all of libcppa builds and all tests pass, but I could only test on OSX with Xcode 4.6's clang.
Would you consider for merging?

Bye,
Andrea