be build - smart interface around cmake invocations

In the previous incarnation of bbuild, the biggest problem in using cmake was the complex commandline arguments that it required. Even though the cmake scripts took care of guiding the user, especially due to the verbose error reporting style the script's code was pretty hard to read.

After all, even for the author it was usually a bit painful to invoke cmake on various systems.

When building, cmake is able to abstract it away, nonetheless there are platform specific differences that have to be accounted for.
## Proposed Solution

Implement a [be](http://byron.github.io/bcore/be/) subcommand which invokes cmake for you with arguments required by bbuild.

It can read required inputs, like the platform specifier, from the commandline or the kvstore (using OverridableCommaneMixin for example).

A session could look like this:

``` bash
$ be build
> [lists all subcommands]
$ be build set blender-plugins
> [configure cmake]
$ be build
> [builds everything with maximum amount of cores]
$ be build clean
> [remove all build results]
$ be build set blender-plugins --reconfigure
> [rerun configuration on given build set, removing for compiler changes for example]
```

It would be great to have a parser for CMake files to allow getting the same information that cmake has, but until then, just configure everything using the kvstore (it's redundant, but we only need a few hints anyway).