Added alternative download url for LanguageTool zip file (jxmorris12#62)

* Added alternate download site capability

* Updated the README with Environment Variable set instruction

* Fixed environment variable documentation to correctly have underscores in name.

* Fixed documentation name of environment variable name to match what is in code

* Fixed logic to handle unset enviornment variable

* fixed length format issue

Author: JohnDLe

README.rst

@@ -66,6 +66,10 @@ If you are using Python 2, you'll need to install 3to2 beforehand::
 
     $ pip install --upgrade 3to2
 
+To overwrite the host part of URL that is used to download LanguageTool-{version}.zip::
+
+    - SET LANGUAGE_CHECK_DOWNLOAD_HOST = [alternate URL]
+
 
 Prerequisites
 -------------

download_lt.py

@@ -8,6 +8,7 @@
 import shutil
 import subprocess
 import sys
+import os
 
 from contextlib import closing
 from distutils.spawn import find_executable
@@ -22,8 +23,10 @@
     from urllib import urlopen
     from urlparse import urljoin
 
-
-BASE_URL = 'https://www.languagetool.org/download/'
+ALT_BASE_URL = os.environ['LANGUAGE_CHECK_DOWNLOAD_HOST'] \
+    if os.environ.get('LANGUAGE_CHECK_DOWNLOAD_HOST') \
+    else None
+BASE_URL = ALT_BASE_URL or 'https://www.languagetool.org/download/'
 FILENAME = 'LanguageTool-{version}.zip'
 PACKAGE_PATH = 'language_check'
 JAVA_6_COMPATIBLE_VERSION = '2.2'