BUG: Avoids install errors caused by lack of pandoc binary.

The pyandoc module throws an OSError of:
"No such file or directory",
when the underlying pandoc binary does not exist.

This error has caused confusion for numerous people during pip
installation.

pandoc is only needed on upload to PyPI to convert to ReST.
So, instead of doing a try/except on the `import pandoc` for all cases,
now checking whether or not 'upload' was invoked when calling setup.py,
so that only maintainers have to worry about installation of pandoc.

By only exercising the pandoc logic when

setup.py

@@ -13,6 +13,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import sys
 
 from setuptools import setup, find_packages
 
@@ -22,15 +23,19 @@ README_MARKDOWN = None
 with open('README.md') as markdown_source:
     README_MARKDOWN = markdown_source.read()
 
-try:
-    import pandoc
-    pandoc.core.PANDOC_PATH = 'pandoc'
+if 'upload' in sys.argv:
     # Converts the README.md file to ReST, since PyPI uses ReST for formatting,
     # This allows to have one canonical README file, being the README.md
+    # The conversion only needs to be done on upload.
+    # Otherwise, the pandoc import and errors that are thrown when
+    # pandoc are both overhead and a source of confusion for general
+    # usage/installation.
+    import pandoc
+    pandoc.core.PANDOC_PATH = 'pandoc'
     doc = pandoc.Document()
     doc.markdown = README_MARKDOWN
     LONG_DESCRIPTION = doc.rst
-except ImportError:
+else:
     # If pandoc isn't installed, e.g. when downloading from pip,
     # just use the regular README.
     LONG_DESCRIPTION = README_MARKDOWN