Made it so transactions will be merged by default instead of rebased.

When synchronizing with multiple repositories, rebasing transactions will cause
history to diverge.  This change makes it so the default behavior is the safe merge,
but rebase can still be specified for cases in which a transaction only deals with local
changes.
Merge pull request #313 from jdeolive/ds_create

Adding option to GeoGitDataStore to enable creation of a repository on demand
Merge pull request #296 from jdgarrett/sync-transaction-fix

Made it so transactions will use merge by default instead of rebase.
fix type of exception expected in LogOpTest that fails after merging 421b49bc
Merge branch 'master' of github.com:opengeo/GeoGit into geoserver