refactor the requirements file parsing

two major changes:

1) re-use the optparse options in pip.cmdoptions instead of maintaining
   a custom parser

2) as a result of #1, simplify the call stack
    from:  parse_requirements -> parse_content -> parse_line
      to:  parse_requirements -> process_line

beyond #1/pypa#2, minor cosmetics and adjusting the tests to match

pip/cmdoptions.py

@@ -434,6 +434,16 @@ def editable():
     help="Don't periodically check PyPI to determine whether a new version "
          "of pip is available for download. Implied with --no-index.")
 
+# Deprecated, Remove later
+always_unzip = partial(
+    Option,
+    '-Z', '--always-unzip',
+    dest='always_unzip',
+    action='store_true',
+    help=SUPPRESS_HELP,
+)
+
+
 ##########
 # groups #
 ##########

pip/exceptions.py

@@ -19,8 +19,21 @@ class DistributionNotFound(InstallationError):
 
 
 class RequirementsFileParseError(PipError):
-    """Raised when an invalid state is encountered during requirement file
-    parsing."""
+    """Raised when a general error occurs parsing a requirements file line."""
+
+
+class ReqFileOnlyOneReqPerLineError(PipError):
+    """Raised when more than one requirement is found on a line in a requirements
+       file."""
+
+
+class ReqFileOnleOneOptionPerLineError(PipError):
+    """Raised when an option is not allowed in a requirements file."""
+
+
+class ReqFileOptionNotAllowedWithReqError(PipError):
+    """Raised when an option is not allowed on a requirement line in a requirements
+       file."""
 
 
 class BestVersionAlreadyInstalled(PipError):