Refactored BasePublisher.print_tree() -> BasePublisher.publish().
HistPublisher prints the file names and number of occurances as a histogram.
<SubPublisher>.print_line() now must handle depth offsets explicitly.
Rewrote GrepTree.walk() to be slightly more flexible.
New GrepTree methods: prune() and set_lines().
Used new GrepTree methods to rewrite the BaseReader.inter().
Made old intersection behaviour available as a flag.

Default piping behaviour is still fast_inter().
Used same optimisation as in intersections to speed up filters/excludes.
Using env in hashbangs to find system python2.7.
Merge branch 'bugs/hashbangs' into develop
@bugfix fast_inter didn't keep track of the line count.
@bugfix fast_exclude didn't keep track of the line count.
Merge branch 'features/exclude_speedup' into develop
Merge branch 'features/more_formats' into develop

Conflicts:
	pygt/publisher.py
Merge branch 'master' into develop
Spelling mistake in CLI arg help.
Bumped version to v0.8.