Add changes to format_taxa_to_js

These changes had the only purpose of changing global constants to be
default parameters as the inputs of the function, as suggested by
@RobKnight.
Change exception handling and header look-ups

Changed the list of headers that's used to verify a value is present
from a primitive to a list, as suggested by @wasade.

Changed exception handling.

Changed option explanation for -x to make it fit in 80 columns.
Add favicon with 2 resolutions 16x16 and 32x32

Seems to be a standard now and having a pull request only for this would
be overkill.
Merge pull request #86 from ElDeveloper/code_review

Code review