fail early if git is too old

git fast-export appeared in git version 1.5.4

Git::FastExport->new now fails if the git binary associated
with the Git::Repository objects is older than 1.5.4
improve linkability of the documentation

by moving all methods under a =head2 heading, and showing code examples below
remove self-links from the documentation
typo fix
Add a more useful $name to the repo object.

This means that translated refs - particularly tags are more
meaningful in the final output and you don't need to know
which order the repos where specified on the commandline.
No need to provide the internal name, build it based on the repo name
remove an unused variable
only do the path translation when an actual directory was provided
Add authored_date to member fields of Block
pick more consistent names for the extra date-related values in Block

the names are now author_date and committer_date
Merge branch 'rgammans/master'

Conflicts:
	lib/Git/FastExport/Stitch.pm
note that as_string() ignores unknown keys in Git::FastExport::Block