Allow to force evaluation points in plot.gbm

This is useful to evaluate the effects of two models on
the exact same data points. An example application is the
evaluation of the confidence around model effects through
bootstraps. For each bootstrap, the data range will be
slightly different and the points at which plot.gbm will
evaluate the effect of the predictor will also be slightly
different, which complicates the computation of a mean+range
of effects afterwards.

The argument is not checked very much and the description in
the help is a little verbose but that's a start.
Improve memory handling code

This code is slightly less horribly leaky.
Improve memory handling

Further improvements to memory handling code.
Move to slightly more adult C++
Use std::vector to handle memory allocation

Use decent RAII pattern to avoid handling memory insofar as is possible.
Improve C++ in package

Here is a miscellaneous collection of changes to begin to convert gbm to
adult C++.  This is far from complete, but is a start.
Improve C++ code
C++ improvements

Part of the ongoing process of C++ tidies.
Remove unnecessary tests

C++ FAQ lite 16.6
Merge pull request #1 from jiho/feature/force_plot_range

Allow to force evaluation points in plot.gbm
Further C++ updates
Merge pull request #6 from pdmetcalfe/c++-sanitize

Some C++ changes
Minor updates to test functions. Previous had absolute mean error rather than mean abs error - replaced with correlation check. Also added some graphical output to be comapred by eye