BUG: TypeError being raised by map_to_grid.

map_to_grid would raise a TypeError when multiple radar objects were passed
and the fields parameters was None.

Bug reported by Marcus van Lier-Walqui, 2013/11/21
Merge branch 'master' of https://github.com/ARM-DOE/pyart
DOC: updated README.rst and INSTALL.rst
MAINT: added sigmet_task_name to metadata of radar object returned from read_sigmet
BUG: time ordering sigmet files does not ignore bad rays
ENH: Better support for vertial pointing scans

* is_vpt function for checking if a radar object is a VPT scan.
* to_vpt function for converting a radar object to a VPT scan.
* plot_vpt method on RadarDisplay class for plotting VPT scans.
Merge pull request #108 from jjhelmus/master

Support for vertical pointing scans and bug fixes