ENH: multi-radar mapping without field data copy.

pyart.io.map_to_grid will now work when multiple radars are passed and
copy_field_data is False.
Initial refactoring of PyGrid object
DOC: fixed link to source code in conf.py
PyGrid object refactoring finished
DOC: PyGrid related documentation
MAINT: cleaning up unused files
ENH: read_rsl can now read WSR-88D Level II data
BUG: distance and Barnes weighting in map_to_grid

* corrected distance calculation to use minkowski distance
* Barnes weighting function corrected
Merge pull request #50 from jjhelmus/master

PyGrid refactor
Updates to gridding functions for MMCG VAP
Merge pull request #54 from jjhelmus/master

Merging back in jjhelmus fork
Revert "Added Jonathan's advice for fixing the bug in rsl to the docs, though a better location might eventually be found"

This reverts commit f03c0b938174da3f975fcaf49ef2b46d286b02ec.
Revert "Corrected naming of RSL variables/ Refactored some hardcoded constants"

This reverts commit 16a4408b8a9beafb2b9aeeab1fd33d413b00fda1.
Merge remote-tracking branch 'upstream/master'
Added Jonathans advice about installation bug with rsl