Add an abbreviation attribute to Charmap classes

[davelab6]

• [x] Add an abbreviation key to Charmap classes
• [x] Parse the Extensis languages.xml file's abbreviation attribute values (example) to this key
• [x] Update the command line so that

pyfontaine --set afk,glat,gviet,al4 font.ttf;

is equivalent to

pyfontaine --set Afrikaans,'Google Latin','Google Vietnamese','Adobe Latin 4' font.ttf;

• [x] Update fontaine/charmaps/*.py to include this key with a suitable value. Example:

from fontaine.cmap import library
class Charmap:
    common_name = u'Full Polish Alphabet'
    native_name = u'Pełny Polski Alfabet'
    key = ord(u'Ł')
    abbreviation = 'plk'
    polishAlphabet = u"AĄBCĆDEĘFGHIJKLŁMNŃOÓPRSŚTUWYZŹŻaąbcćdeęfghijklłmnńoóprsśtuwyzźż"
    glyphs = map(ord, polishAlphabet)