Add -DUSE_UNSTABLE when making an unstable build.

 - This is not echoed in the CMake summary as it is set in the
   unstable directory, not through DYNAMIPS_DEFINES in the top
   level. This is to prevent the -DUSE_UNSTABLE being added when
   producing a 'dual' stable/unstable build (aka both)

Fixes #55
Bump version to v0.2.14.
Bump version to v0.2.15-dev
Merge https://github.com/GNS3/dynamips