PEhash module

Added PEhash module to calculate the PE hash of a open file or of all
files in the project and check for any files that have the same PEhash
and print these.
PEhash functionality in PE module

Moved the PEhash module to a sub option in the PE module for easier
access. Added compare functionality and printing functionality for all
files
Package bitstring

Added the bitstring pip to requirements and removed some newlines in
the pe file
Merge pull request #141 from nheijmans/master

PEhash module
Improved pehash
Fix error when 'Characteristics' field is single byte
Merge pull request #143 from jahrome/fix_pehash

Fix error when 'Characteristics' field is single byte
Fixed help message
Fixed indentation in yarascan
Updated CHANGELOG
Small updates