Allow alternate configuring of tool path & environment.

Paths to toolsets (such as for msvc) where read from the environment.
This then required multiple executions to change between tools to allow
alternate environments to be selected prior to execution of task.

Allow the specification of tools path to allow for multiple executions
in a single environment such as:
 * 32 & 64 bit compilation,
 * toolset versions.

[JES: fixed formatting, naming]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'Pathing'

This closes the pull request 11:

https://github.com/maven-nar/cpptasks-parallel/pull/11

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>