BUG: masked values no longer appear in grids

Masked values no longer are allowed to 'leak' through into Grids which are
produced from the `map_to_grid` function when **copy_field_data** is True.
The documentation for the `map_to_grid` function has been updated to better
explain the **copy_field_data** parameter and provides a method for preventing
masked values from leaking through to the grid when **copy_field_data** is
False.

Closes #105
ENH: radar_n_ keys in Grid metadata

Grid objects produced from the `grid_from_radars` function have
radar_n_lat, radar_n_lon, radar_n_alt, radar_n_instrument_name
keys in the metadata dictionary where n is the radar number
(starting from 0).

This addressed some on the concerns in issue #107.
WIP better default qrf_func
ENH: better default roi_func in map_to_grid

* Renamed `qrf_func` to `roi_func` in map_to_grid.
* Three build in `roi_func` build in, 'constant', 'dist' and 'dist_beam',
  'dist_beam' is the default for backwards compatibility.
* The 'dist' and 'dist_beam' built in RoI functions now take into
  account the offset between the nearest radar and the grid when
  computing the RoI.
* grid_shape and grid_limits are now required parameters to the
  map_to_grid function.
* grid_shape, grid_limits, and roi_functions have dimension ordered as
  z, y, x, which matches the output grid shape. Internal this order is also
  used.
* Better documentation for all of the above.
DOC: fixed map_to_grid docstring
EXAMPLES: updated mapping examples
TST: unit tests for grid_mapper.py module
Merge pull request #115 from jjhelmus/mapping_enh

ENH: Various enhancements to mapping and the Grid object