Added gitignore
Add tanproj module to handle tan-plane transformations

Older plots used tan coordinates. This module provides functions to
transform between tan and equatorial coordiantes.
Script to create JSON file with narrowband fields

scripts/prep_narrowband_fields.py writes the json file. It provides
a dictinary of field name, and box in equatorial coordinates. Both
archive CFH12K fields and new MegaCam fields are included. Eventually
these datasets will be in the imagelog db.
Refactor polygon union function into polytools

This refactoring makes these function available to more general mapping
procedures.
Support for narrowband field mapping

- new script to plot narrowband fields (archive/observed/unobserved)
  see scripts/narrowband_map.py
- Added plot_narrowband_fields() method to Andromap
- Include narrowband field position in JSON file