Fix version in pkg-config file, also change bzr version format slightly

Old bzr version format: bzr-1234
New bzr version format: 3.2.1-bzr-1234

Version spoofing with the BITLBEE_VERSION also changed slightly - it
does not need quotes around the version number anymore for it to compile
correctly.
Add git version string support to configure
Merge pull request #13 from dequis/patch/pkg-config-version-fix

pkg-config version fix