Dr. Memory Framework: combine all the drmem-based DR Extensions

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on October 17, 2012 12:12:42_

We currently have 4 DR Extensions that we plan to leave in the Dr. Memory
repository:
- DrSyscall = issue #822 + DrCallstack = issue #823 + DrMalloc = issue #824 + Umbra = issue #825 My proposal is to have them all share the same versioning scheme and
packaging. We could package up DynamoRIO with them for one-stop shopping.

Xref various discussions involving DrSyscall for file layout and top-level
design.
Xref DRi#949 for 3rd-party extension support via find_package() to better
support bundles of Extensions like this.

This issue covers adding the skeleton framework for the DRMF.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=1056_