replace decade old pyasn1 crypto impl with working asn1crypto

For some APKs, get_certificate() was returning a different result than
apksigner and keytool.  So I just took the algorithm from androguard, which
uses asn1crypto instead of pyasn1.  So that removes a dependency as well.
asn1crypto is already required by androguard.

The original get_certificate() came from 6e2d0a9e1

setup.py

@@ -93,14 +93,13 @@ setup(
     install_requires=[
         'appdirs',
         'androguard >= 3.3.5',
+        'asn1crypto',
         'clint',
         'defusedxml',
         'GitPython',
         'paramiko',
         'Pillow',
         'apache-libcloud >= 0.14.1',
-        'pyasn1 >=0.4.1',
-        'pyasn1-modules >= 0.2.1',
         'python-vagrant',
         'PyYAML',
         'qrcode',