Add functions to add environment variables
Conform to Python idioms
Merge pull request #60 from kyptin/environment

Add functions to add environment variables
Bump minor version
Merge pull request #61 from elliot42/master

Bump minor version
Add context manager for environment variables

This allows you to temporarily have a context with different
environment variables set/unset.
Merge pull request #62 from elliot42/master

Add context manager for environment variables
Enable excluding directories from creation

Some directories in our abstract tree we want to be able to
address/refer to, but not necessarily create when we create the tree.
This commit enables `mcv.dir.mktree` to exclude creating nodes based
on a predicate given at creation time--by default it defaults to
creating all the nodes given.
Merge pull request #63 from elliot42/master

Enable excluding directories from creation
Enable git.clone using implicit SSH keys

Prior to this commit you could only git.clone using explicit paths to
SSH keys; this commit allows you to perform the operation using the
implicit SSH keys already in the environment.
Merge pull request #64 from elliot42/master

Enable git.clone using implicit SSH keys