Add GuildDetails lookup.

Adds a guild lookup by name.  To test:

from guildwars2api import client

api = client.GuildWars2API()
api.guild_details.get('Colorful Metaphor')


Should return something like this:
Out[3]:
{u'emblem': {u'background_color_id': 473,
  u'background_id': 20,
  u'flags': [],
  u'foreground_id': 146,
  u'foreground_primary_color_id': 4,
  u'foreground_secondary_color_id': 93},
 u'guild_id': u'35131675-2AD7-4897-9545-BAC9539D7C95',
 u'guild_name': u'Colorful Metaphor',
 u'tag': u'CM'}