Boost 1.56

In Boost 1.56 a new boost module `core` was introduced.[1] This broke usage of `core::`. With this fix the ambiguous naming is avoided. I assume it is just a temporary fix until someone brings up a smarter solution not modifying that many files.

I tested RStudio in desktop mode build as release. Since this fix only contains some renaming `namespace core` to `namespace rstudiocore` and `core::` to `rstudiocore::` it should be downward compatible. Renaming was just the easiest way of not breaking something along the way.

[1] http://www.boost.org/users/history/version_1_56_0.html