Don't require clean repo by default

[jonahgraham]

Hide tycho-buildtimestamp-jgit's requirement of clean git tree behind a profile that is activated on build machines, but not required on dev machines. This relates to #165 in that doing compile with maven outside Eclipse is a pain because of it.

[jonahgraham]

It may make sense that build-prefix=='v' requires clean, and others do not as released version should be v prefixed.