Add g:makeshift_root and :MakeshiftBuild

To allow building when the Makefile is in a parent directory
Allow for passing arguments to :MakeshiftBuild
Fix typo
Add help for g:makeshift_root and :MakeshiftBuild
Update README for v0.7
Bump version to 0.7
Merge pull request #4 from johnsyweb/determine-directory

Works when SConstruct file is not in the current directory