Make bitcoin include files modular / Qt4 compat #225

To be able to make an external GUI, I am working on modularizing bitcoin into a library. This push request is basic code plumbing, 100% no functional or build system changes.

• Put guard statements around header files.
• Removed macro foreach: conflicts with Qt4 keyword foreach, replaced with BOOST_FOREACH.
• Prefix C++ stdlib structures and functions with std:: in headers; "using namespace" in header files is generally frowned upon because it 'infects' other files included after it. These are moved to the implementation files.
• Modularity: base48.h and most other header files can now be included without the other shebang (useful for linking external GUI to bitcoin core, part of GUI separation). The include files that need each other, now include each other.

You can return to a previous commit in git with git reset HEAD~ --hard, and push that again using git push origin -f master. The pull request will automatically adapt.

Thanks! now it's back to one commit (uhm, sorry for pressing the open/close buttons, that goes too easy!)

ACK, looks good to me. We need to move off of headers.h and this looks like a good first step (plus if it helps with QT compat, and we do eventually move there, Im in favor)

Compiles and runs fine. This is a nice step towards cleaning up the code. ACK

ACK, good idea. I'm going to pull.