Make HTML transformation dependencies optional

pip install emails        - lightweight, only message building and sending
pip install emails[html]  - full install with HTML transformer (lxml, cssutils,
                            premailer, chardet, requests)

Move heavy dependencies (lxml, cssutils, premailer, chardet, requests) from
install_requires to extras_require['html']. Make requests import lazy in
utils.py so that `import emails` works without it.

Author: lavr

emails/utils.py

@@ -15,8 +15,6 @@
 from email.header import Header, decode_header as decode_header_
 from email.utils import parseaddr, formatdate, escapesre, specialsre
 
-import requests
-
 from . import USER_AGENT
 from .exc import HTTPLoaderError
 
@@ -270,6 +268,7 @@ def __init__(self, _subtype='mixed', boundary=None, _subparts=None, encoding=Non
 
 
 def fetch_url(url, valid_http_codes=(200, ), requests_args=None):
+    import requests
     args = {}
     args.update(DEFAULT_REQUESTS_PARAMS)
     args.update(requests_args or {})

setup.py

@@ -127,7 +127,10 @@ def find_version(*file_paths):
               'emails.packages.dkim'
              ],
     scripts=['scripts/make_rfc822.py'],
-    install_requires=['cssutils', 'lxml', 'chardet', 'python-dateutil', 'requests', 'premailer'],
+    install_requires=['python-dateutil'],
+    extras_require={
+        'html': ['cssutils', 'lxml', 'chardet', 'requests', 'premailer'],
+    },
     zip_safe=False,
     classifiers=(
         'Development Status :: 4 - Beta',