Start to avoid using globally defined namespaces

The basic idea: use "uncommon" namespaces inline instead of globally.

This PR is rather tolerant and does not remove global usings, but I think untangling dependencies etc. would be the next step.

The following are available as well - each of them for a specific namespace, but I felt like this would be an overkill to begin with:

- std: [mcore-namespace-std](https://github.com/dexX7/bitcoin/compare/mastercoin-MSC:mscore-0.0.8...mcore-namespace-std)
- json_spirit: [mcore-namespace-json_spirit](https://github.com/dexX7/bitcoin/compare/mastercoin-MSC:mscore-0.0.8...mcore-namespace-json_spirit)
- boost: [mcore-namespace-boost](https://github.com/dexX7/bitcoin/compare/mastercoin-MSC:mscore-0.0.8...mcore-namespace-boost)
- leveldb: [mcore-namespace-leveldb](https://github.com/dexX7/bitcoin/compare/mastercoin-MSC:mscore-0.0.8...mcore-namespace-leveldb)

What I'd like to avoid. mixed use of global/local namespacing. E.g. `std::string` sometimes, and sometimes only `string`.