Toolset modules

If your extensions will be used only on one project, they can be placed in a separate .jam file that will be imported by your project-root.jam. If the extensions will be used on many projects, users will thank you for a finishing touch.

The using rule provides a standard mechanism for loading and configuring extensions. To make it work, your module should provide an init rule. The rule will be called with the same parameters that were passed to the using rule. The set of allowed parameters is determined by you. For example, you can allow the user to specify paths, tool versions, and other options.

Here are some guidelines that help to make Boost.Build more consistent:

The init rule should never fail. Even if the user provided an incorrect path, you should emit a warning and go on. Configuration may be shared between different machines, and wrong values on one machine can be OK on another.
Prefer specifying the command to be executed to specifying the tool's installation path. First of all, this gives more control: it's possible to specify
```
/usr/bin/g++-snapshot
time g++
```
as the command. Second, while some tools have a logical "installation root", it's better if user doesn't have to remember whether a specific tool requires a full command or a path.
Check for multiple initialization. A user can try to initialize the module several times. You need to check for this and decide what to do. Typically, unless you support several versions of a tool, duplicate initialization is a user error. If the tool's version can be specified during initialization, make sure the version is either always specified, or never specified (in which case the tool is initialied only once). For example, if you allow:
```
using yfc ;
using yfc : 3.3 ;
using yfc : 3.4 ;
```
Then it's not clear if the first initialization corresponds to version 3.3 of the tool, version 3.4 of the tool, or some other version. This can lead to building twice with the same version.
If possible, init must be callable with no parameters. In which case, it should try to autodetect all the necessary information, for example, by looking for a tool in PATH or in common installation locations. Often this is possible and allows the user to simply write:
```
using yfc ;
```
Consider using facilities in the tools/common module. You can take a look at how tools/gcc.jam uses that module in the init rule.