This commit allows different branches of WebRTC to be built.

Main differences with implementation in 730b7fd:

• WEBRTC_BRANCH_HEAD now only require the branch number e.g. 58

• WebRTC is now cloned during the main configure rather than in the download/configure steps of the webrtc-src external project. This allows to find out which branch is being built when building with the -DWEBRTC_REVISION option.

• LIBWEBRTC_VERSION is now suffixed with +${WEBRTC_BRANCH_HEAD}-${WEBRTC_REVISION} e.g. 1.0.0-9-g13f0747+60-6294a7eb71c891e9ea41273a7a94113f6802d0da

• Part of the code is now conditional on ${WEBRTC_BRANCH_HEAD} value

Known issues (which are not regressions):

• WebRTC branch 59 build fails on Windows x64 because of missing symbols (libjpeg-turbo symbols built with yasm). No path is shown for those objects when they are built and I don't have a Windows setup to investigate further.
• WebRTC branch 61 build fails on Linux (at least x64) because of a C++ ABI incompatibility.

  /home/travis/build/mayeut/libwebrtc/sample/main.cpp:48: undefined reference to `rtc::Thread::SetName(std::string const&, void const*)'
  /home/travis/build/mayeut/libwebrtc/sample/main.cpp:49: undefined reference to `rtc::Thread::SetName(std::string const&, void const*)'
  

  This symbol is defined as _ZN3rtc6Thread7SetNameERKSsPKv in main.o and as _ZN3rtc6Thread7SetNameERKNSt3__112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEEPKv in libwebrtc.a (seems like libstdc++ vs libc++)

Some builds are visible here (branches 57 to 61):
https://travis-ci.org/mayeut/libwebrtc/builds/266494102
https://ci.appveyor.com/project/mayeut/libwebrtc/build/1.0.21