rez.config: rename 'env' to 'namespace' since this is a more accurate description of its purpose.
rez.rex: change ExactVersion formatting to use "new-style" python formatting.
- remove ExactVersion.part(0) in favor of .parts[0]
- remove ExactVersion.thru(2) and replace with a "new-style" format specifier (e.g. '#.#'). for more info see Issue #48 (https://github.com/nerdvegas/rez/issues/48)
rez.packages: rename 'path' argument to 'metafile'.
Homogenize Package classes, particularly class instantiation:
- create a new BasePackage class
- make ResolvePackage a sub-class of BasePackage
- rework __init__ args so that argument order is more similar between sibling classes
rex: rename 'pkgs' to 'resolve'. provide 'request'.  rename ResolvedPackages to Packages, since it can be used to hold a list of any type of package.