Change `ssh` agent forward class & close when done

Fixes #562
Cut 1.3.7
Merge branch '1.3' into 1.4

Conflicts:
	docs/changelog.rst
	fabric/version.py
Cut 1.4.2
Clarify relative root file path of lcd().

Fixes #634
Merge branch '1.3' into 1.4

Conflicts:
	docs/changelog.rst
Update MANIFEST with more test/doc contents.

Fixes #640
Merge branch '1.3' into 1.4
Make Sphinx git hook degrade gracefully.

Fixes #645
Merge branch '1.3' into 1.4
Add failing test re #649
Don't swallow exceptions in child processes.

Also makes parallel execute() honor warn_only.

Fixes #649
Changelog re #649
Making a note about using multiple `with`s

Official Python documentation has this feature documented here:
http://docs.python.org/reference/compound_stmts.html#the-with-statement
Formatting tweaks re #651
Append to note re: contextlib.nested on Python 2.5. Re #651
Changelog re #651
Merge branch '1.3' into 1.4

Conflicts:
	docs/changelog.rst
Show valid commands when handling invalid names.

Fixes #652
Merge branch '1.3' into 1.4