Add copyright notices to source files missing them. This was reported
by Matthew Dempsky.

I'd like to redo all the copyright headers at some point because the
"AUTHORS" thing is not useful (that's what git blame is for!). I'd also
like a textual description of what the code in each file is for/does.

I think following the LLVM copyright header text style would be a good
idea.
Add .clang-format style file and handy script to run from project
root to format all STP sources (apart from external projects) with
clang-format.
Clang-format the code base. STP's code was a big mess of diferent
styles. This unifies the style based on our .clang-format style file.

This might make using ``git blame`` a little harder. You can use the
``-w`` to ignore whitespace changes which works for many lines that
have been changed by this commit but not all. For lines where ``-w``
does not work you need tell git blame to look at commits before this
one. For example

$ git blame XXXXXX^ include/stp/AST/AST.h

where XXXXXX is the commit hash of this commit.