Scons-tool-gnuplot

- use Detect() instead of WhereIs() to detect gnuplot
- fix typo 'BUILDER' -> 'BUILDERS' in gnuplot/__init__.py

- Use curl instead of hg to download dependencies.
- Download testing framework from main scons repository. The dirkbachle's one
gone away.

- Fixed bug with incorrect order of targets returned by emitter (github issue
2).
- Revised user manual and tests to reflect the interface change (chdir ->
gp_chdir) - see below.
- Fixed possible bug, where the usage of SCons built-in chdir parameter could
broke parallel builds (the -j flags). We now use our own 'gp_chdir' keyword
parameter instead of SCons built-in 'chdir' parameter.

- Revised user manual
- GplotGraph converted from pseudo-builder to true builder

- Initial release.