New Shorten-Tweet Function

# Shorten-Tweet
# Function for the PowerShell MyTwitter module
# You sometimes want to replace words in your tweet to comply to max
tweet length of 140 characters
# Date: 09/10/2014
# Author; Stefan Stranger
# Version: 0.1
# Changes:
#  (0.1) = initial version
# ToDo: - speed up performance. .Net class is faster but does not do
case insensitive replace.