Fix luigi.s3 and luigi.webhdfs default imports

Don't fail when importing luigi.s3 or luigi.webhdfs if deps
aren't satisified. This improves the ability to generate api
docs for these modules when required packages aren't installed,
and it is in line with the pattern used in luigi.{hdfs,hive,postgres}
and other places.

luigi/s3.py

@@ -17,11 +17,9 @@
 import os.path
 import random
 import tempfile
+import warnings
 import urlparse
 
-import boto
-from boto.s3.key import Key
-
 import configuration
 from ConfigParser import NoSectionError, NoOptionError
 from luigi.parameter import Parameter
@@ -31,12 +29,20 @@
 from luigi.task import ExternalTask
 from luigi.format import FileWrapper
 
+logger = logging.getLogger('luigi-interface')
+
+try:
+    import boto
+    from boto.s3.key import Key
+except ImportError:
+    logger.warning("Loading s3 module without boto installed. Will crash at runtime if s3 functionality is used.")
+
+
 # two different ways of marking a directory
 # with a suffix in S3
 S3_DIRECTORY_MARKER_SUFFIX_0 = '_$folder$'
 S3_DIRECTORY_MARKER_SUFFIX_1 = '/'
 
-logger = logging.getLogger('luigi-interface')
 
 class InvalidDeleteException(FileSystemException):
     pass

luigi/webhdfs.py

@@ -5,10 +5,18 @@
 """
 import datetime
 import configuration
+import logging
 import os
 import posixpath
 import urlparse
-import whoops
+
+logger = logging.getLogger("luigi-interface")
+
+try:
+    import whoops
+except ImportError:
+    logger.warning("Loading webhdfs module without whoops installed. Will crash at runtime if webhdfs functionality is used.")
+
 
 
 def get_whoops_defaults(config=None):
@@ -60,10 +68,6 @@ class WebHdfsClient(object):
     use the client, you must specify `namenode_host` and `namenode_port` in the `hdfs` section of
     your luigi configuration."""
 
-    def __init__(self):
-        "Configures _homedir so that we can handle relative paths"
-        self._homedir = get_whoops("/").home()
-
     def _make_absolute(self, inpath):
         """Makes the given path absolute if it's not already. It is assumed that the path is
         relative to self._homedir"""
@@ -72,7 +76,7 @@ def _make_absolute(self, inpath):
         if scheme or posixpath.isabs(path):
             # if a scheme is specified, assume it's absolute.
             return path
-        return posixpath.join(self._homedir, path)
+        return posixpath.join(get_whoops("/").home(), path)
 
     def exists(self, path):
         """Returns true if the path exists and false otherwise"""